We did this cruise last February and loved it. It was our first cruise and didn't know what to expect, we went with our DSs, aged 15 and 16.
It didn't have a booze cruise feel to it, though there are lots of different bar areas, but didn't see anyone that seemed drunk. There's an Irish bar called Brodies which showed sports so occasionally was a bit of shouting when one team scored but I certainly didn't think it was 'rowdy'.
Some people say Iona is too big, it holds over 5000 people but again (having nothing to compare it to) I didn't ever feel it was too crowded. There's the buffet restaurant which was busy at peak times but I think all buffet restaurants have a slightly chaotic feeling to them!
There are also the MDRs (main dining rooms), there are four of them and each meal time all the MDRs serve the same menu, but the menu changes each night so if you go to a different one it's like eating in 4 different restaurants.
They have music and comedians on every night, and there was only 1 night we couldn't get a seat, but we got there almost as the act started (the ventriloquist guy from BGT!) so could have planned better.
I have 2 more cruises booked now, I thought I'd enjoy it and did, loved waking up in a new place and loved the itinery of this cruise. Bruges was our favourite place.
Might be worth you pricing it direct with P&O, we got loads of free on board credit, not sure if you get that with Iglu?
Also another tip, take sandals/flip flops/sliders etc. I thought as it was winter, to northern Europe I'd be cold, so only had my winter boots, gym trainers and heels for the nights with me. On the sea days and when wandering about the boat during the day I constantly had boiling feet, drove me potty. The boat is warm inside even when chilly outside. I should have thought about that!