Lucky Happy Height Day tester here - sorry for the shamefully late response and needing to be reminded. 
First, thank you very much for the brilliant opportunity. We all had a blast! We were a group of four: dh and I, and two dc aged 9 and 6. We'll certainly be back for more when Angry Birds World opens, and in fact the whole family had such a great time that we have subsequently promised the dc that we'll buy Merlin passes when the Thorpe Park passes run out. We've already done two Legolands, Sealifes, London Eye and the Blackpool Tower and Madam Tussauds in the past, and can't wait to add Thorpe Park to our regular trips. We'd always been chary of forking out that amount, but it will definitely be worth it. The long trip up to visit my inlaws in Lancashire now looks a lot more bearable too if we can stop off.
I'm the only one who'd been to Thorpe Park before, but that was in the 80s as a school kid, in the days of Space Station Zero. The two experiences are not comparable! Dd had been to Chessington with a friend recently, however, and so had high expectations. She declared Thorpe Park 'a million times better than Chessington' (despite absence of posse of friends - and presence of irritating small brother).
The cuddly Angry Bird was a massive hit too, so thank you very much for that. We've already had to promise to visit Angry Bird land when it opens and win/buy one for little bro...
Said small brother had a whale of a time as well, and in fact was a lot bolder that his older sister (although possibly safe in the knowledge that he wouldn't be allowed to go on the big rides he envied her). He rode the Rocky express FOUR times and was on first name terms with the very friendly guy working it by the time we dragged him away.
The Flying Fish roller coaster was also really popular with both of them (was this really Space Station Zero once? It felt a bit tamer than I remember that being), and the two of them went on seven times between them. I liked the ease of getting on, especially with the fast track passes. This was a ride which seemed not to gather huuuge queues, so when we return with ordinary passes we'll definitely go on this ride - it seems great value for the waiting time. Again, the staff working it had a fabulous rapport with children (red-haired young chap outside, and brunette young woman in the booth), and made every circuit fun.
The park had lovely staff all round, actually, who were chatting to our kids and knowledgeable about their rides. It was clear that they all knew the park very well, and had been on the rides, and they happily warned us against some until we felt bolder, and recommended others!
Thanks to the nice people at Nemesis Inferno, dd went on it, even though she was terrified and only agreed to think about it as we pointed out she had to review at least one of the 'big rides'. The man at the entrance allayed her fears and encouraged her to give it a go by describing what it felt like (literally yard by yard, it felt like!), and when she did go on, she absolutely yelled with excitement and wanted to go straight back on. Then, of course, it broke down, so that was the end of that. 
Nemesis Inferno is a great introduction to big rides for a nervous 143cm kid, though, so well recommended, Thorpe Park chap!
No one was willing to take on Saw or Stealth or the Swarm, or indeed Tidal Wave. We all agreed that it was the sheer height that put us off. So dd would like more rides like Nemesis and Colossus for vertigo-avoidance. Or she could just get taller and braver, I suppose!
X was a highlight for both dd and ds. The queues were terrible, though, even with fast track for the outside queue, and the music was a little loud for all of us. Everyone was willing to wait for a second turn, though, and we would have done if it hadn't been too close to closing time (we spent all day from opening 'til closing time and still didn't manage everything we wanted to do).
The Storm in a Teacup was a great old classic and enjoyed by all (we got some good ellipses going with judicious use of the central wheel).
Loggers Leap also met with general family approval, as did Depth Charge.
Colossus was a hit with dh and dd. 'Not as scary as it looks', apparently, but over very quickly, leaving them wanting another go but not willing to endure the queue again.
We'll avoid Mr Monkey's Banana Boat next time, though - Mr Monkey needs his sign repainting rather badly, as it looks a bit shabby, but more importantly the ride itself was just feeble, and could perhaps be sign-posted as suitable for the nervous (along with the carousel) as it was a bit tame even for our 6 year old. Luckily, though, we shared our go with a group of visitors determined to wring a scream out of every swing, and their mock-terror made it quite fun. No one wanted to go on ever again, mind you. Sorry, Mr Monkey.
We also found no hand-dryers working in either of the lavs we used. This was pretty poor, given the park had only been open three days since the start of the season, but I expect it could have been easily rectified. There are so many brilliant hand-dyers around that I think we've come to expect them as standard. The kids liked the cool circular sinks, though, and so did I. Very hygienic and appealing.
Saddest was the filthy state of the water, especially given how early in the season/day it was. It was particularly bad on the pond to the right of the walkway up to Rumba Rapids *, where there was lots of detritus and rubbish lazily floating about on the water. Given the decent nick of the pedestrian areas, it would be nice if the landscaping around them were as clean and pleasant. Visit Legoland Billund to see how to do it. We saw a couple of ducks, but little in the way of interesting planting. Some one with a net on a long stick/a pair of waders could have made a lot of difference very quickly.
- Nice ride, but a bit 'was that it?' at the end.
We also had genuine sobs on Storm Surge because of the water pistols. On our third time around (it's a great family ride), a couple of teens were using the pistols, and focused on poor dd (aged 9, so an odd choice for a couple of mid-teens), with the result that she got water full in the head/face/neck for more than a minute (they ignored other groups). It felt pretty mean, and we couldn't avoid it (unlike Legoland Windsor where the water-guns are free, but shoot pulses of water not a solid jet). She was in tears for quite a while after, and she's a pretty resilient soul, having a younger brother. I'd like this to be set up so there's only a brief opportunity to get each group as it passes.
Having said that, it was good to see some security guys handling another situation firmly and efficiently but discreetly - the dc didn't even notice as we strolled past. Nicely done, the adults thought.
Dd also loved her special certificate - it is a fabulous idea and is now framed and on her wall, bless her. Thank you! It is a lovely idea if it's a regular thing.
The food we had (ribs, a burger, drinks, and waffle-type chips at the Calypso BBQ and a hotdog at the hotdog stand) was all tasty and generous of portion, and the people serving friendly and helpful. Without the money vouchers we'll bring our own drinks and snacks, though, as the prices were a bit steep, and just buy one hot meal each.
Final thoughts are:
- smoking needs to be in designated areas and well policed, as it's a bit grim to have so many people waving lit cigarettes around, especially in queues where you can't escape it
- Having alcohol on sale doesn't seem like a good idea at all, really, but I am a bit of a worrier
- the passes are very good - but 75 for a day one?! That's painful (when a year pass is presently 48). I didn't really look in detail at the list of fast track options near Depth Charge (which is also great, btw), but I got the impression there was no option to just buy a booklet of say 15 fast track upgrades and use one each time you felt like it. This would be good (though as I say, may be available and I just didn't spot it).
- Our best recommendations for those just over 140 cm are Colossus and Nemesis Inferno. Colossus is particularly fun as a spectator, too.
- We have raved about our day to loads of people, so yes, we would recommend Thorpe Park to our friends!
Overall, thank you very much for a brilliant day which everyone enjoyed. We will be back!