I've been a couple of times and thoroughly enjoyed it - my only minor complaint would be the queues which seem to be pretty long much of the year round.
I prefer the rides that give you a bit of an adrenaline rush - Nemesis is probably my favourite. Air and Oblivion also really got my pulse racing, though I'm not great with heights so it took a bit of persuading to get me on either of those!
Additionally there are plenty of gentler rides for those who don't appreciate being hurtled around a track at excessive speeds. The Runaway Mine Train and Flume are a good bet for families, whilst the River Rapids are worth a look for those who like water.
I was fortunate enough to get good weather on both of the occasions I went, though with plenty of sideshows, restaurants and rides, Alton Towers is probably the top theme park in the UK.
I went once I think and was
I went once I think and was seriously unimpressed with the queues. I lived about 15 minutes drive from Drayton Mannor Park and so as a kid we went there a lot, and it just seemed a big long car ride for doing something that we could be doing far closer to home and at lower cost.