I've just done my first business class flight long haul and my first Premium long haul too. I've done lots and lots of long haul in economy.
I don't think it's worth the extra for business class if I'm paying, to be honest, especially if you're booking at full price. (e.g. paying £4k for a 10 hr flight, rather than the £1.5k that you can sometimes get them at).
The food was nice in business, and it was nice to be able to lie flat to sleep but was it £3.5k worth of nice? Not at all. I tried very hard to drink my way through the extra money, but it was a struggle. The flat bed wasn't that comfortable - I slept but not much.
I was, however, very impressed with premium. Significantly comfier seats and much better food (v standard economy). For a day flight, I wouldn't waste my money on business, but would definitely pay to fly premium.
Don't forget, you can buy reasonable lounge access for <£50.