Well, here's a few of my thoughts, having done the summer one twice.
TG is the main reason I run... I'm not a good runner, but you have to be able to do it to do TG, and it's all good for me!
I LOVE the assault course type stuff. There's something so much fun about getting wet and muddy, challenging yourself and doing things you normally wouldn't, like (for me) 'walking the plank' and jumping into water - I dread that bit! They're building a higher plank platform for this winter. It looks horrendous. I think they're keeping the old one as well,so you have a choice of heights to jump from. I really hope so. Maybe in the summer I'll go for the high one...
There's also a great atmosphere at TG. It's not competitive - well it probably is if you're one of the elite ones at the front, but not for the rest of us - and there's always someone who'll give you a hand or a friendly shove if you get stuck. It's a great day out even if you're just watching, which I did last winter.
If you look on the TG website, it does look incredibly scary. It isn't as bad as they make out - at least, that's true for the summer one. I think there'll be quite a difference between that and the winter one though. The main issue in the winter is the cold, so it does require cold weather training, getting yourself used to freezing water etc etc. Which reminds me, I'm due another run in the river...
There are another couple of broadly similar races I'd like to have a go at. One's called the Grim, and the other one is something military - basically they open some forces' assault course to the public for a day. They're both down south - one's in Aldershot and I forget where the other is. They sound less 'assault course-y' than TG though - more natural obstacles I think. If anyone fancies a go, let me know