I'm wondering about thermarests ... [snip] ... thoughts?
I think most of it's been said, foam mats work OK but they just aren't as plush as a Thermarest. Like many others here I've used foam mats, and I've used Thermarest brand thermarests for decades. I've never had to repair one, even those I've had for over 20 years. Thesedays mine get used less often than I'd like, maybe 20-30 nights a year - it varies - but I never even think about a puncture except when I'm setting up camp. Then I make sure there's nothing likely to puncture me in the night, such as thorns or cutlery.
Are they worth it?, you asked. Well it depends how much you value your comfort and your money, perhaps how much of them you're used to. If one of mine got a puncture I'd replace it immediately, so I'd say an unqualified yes, but I don't know your stance on the comfort/money balance. IMHO they're one of life's little luxuries, like having a pottery mug instead of one made of stainless steel.
There are just a few of issues that seems to have slipped by.
First issue: I have two, both 3/4 length (I wouldn't bother with longer for reasons that have been covered) but they are quite different. One folds lengthways once before it's rolled up, the other doesn't. I usually take the one which folds first if I'm on the bike because it's easier to find a space for it, but it's much more awkward to get rolled up and back into its pack than the one which doesn't fold. If you're going to be setting up camp in some place and staying there for a good while then it probably doesn't matter, but if you're setting up in a different spot every night, well, to me at least it taints the morning experience ever so slightly. I find breaking camp the least pleasurable bit of the whole thing anyway, that's just one of the things I don't like about it, but maybe I'm obsessive.
Second issue: They're slippy! I have no problem when I use one in my bivi bag but if I use a tent I daren't camp on even the slightest slope or I'll be sliding off it all night. In my darker moments I've seriously considered sticking some safety pins through the danged thing so my sleeping bag will stay on it. It's worse if you use a bag which has a man-made fibre outer, and if you're going to be sleeping in sub-zero conditions I expect you will be. I can't remember the last time I saw a cotton one anyway. My SO made a cotton 'shroud' which can go inside my bag or outside it, that can help with some friction but it weighs nearly as much as the sleeping bag and it's as bulky so it rarely goes out with me on the bike.
Third issue: You shouldn't leave them packed for any great length of time. You need somewhere to have them unrolled and inflated if they won't be in use for a while. They take a while to inflate themselves, but as has been said don't be tempted to help by blowing into them.
Have fun, and don't be afraid to take a couple of hot water bottles.
Foam pads are cheap especially if you already own one. I would love a thermarest but that money is going towards "free" time. 
Check out some of the videos on Youtube for "jumping cactus". I can't find one without profanity, so no link...
