I ran the Rome marathon with Team in Training a few years ago, and it was a great experience for me -- they helped motivate me to something that just a year or so earlier I never though I'd be able to do. On top of that, I raised almost $5,000 for Leukemia research (If you know don't anything about cancer treatment, the LLS and cancer researchers have really done remarkable things).
That being said, it is a huge commitment. Not only does it take a lot of time to train -- by the end of the marathon training, I was taking 4hr training runs on Sundays plus 1 to 2 hr runs 3 days a week (the runs weren't THAT long, I'm just THAT slow). Plus, you have to commit to raising a good chunk of change.
The TNT crew are all good. I had the unfortunate position of being in a group located about an hour from me, so I didn't get the full support structure that a lot of other people from more populous groups did. I wasn't able to make it to a lot of the training runs with them because of the distance (a 7:30 training run an hour away was tough for me to do).
During the event, the support is fantastic, with people cheering you on throughout the race, a great group of people to go with. I just had a blast torturing my body for 26.2 miles. I'd suggest doing like me and taking advantage of the opportunity to take on one of the more exotic events - the Dublin Marathon is supposed to be great and the Rome event was fantastic. If I decide to do a century sometime, I'd like to do Tahoe though -- that has to be one of the most beautiful.
I'd totally do it again though... as soon as I get the nerve up to ask everyone I know to donate again. That was the toughest part for me.
If you decide to go for it, post it up here and I'll start you off with a donation.