I did my first triathlon with TNT and had a great time with it but there are a few warnings. The race was St. Anthony's at the end of last April and the training program they offer is really good. They build you up for 3 or 4 weeks and provide a recovery week. Everything is based on time rather than distance. This allows for tougher training days outside (I live in Chicago and there are some TOUGH days during the winter and early spring) and people of all paces to follow the same program.
Raising the money wasn't that tough and goes easier if your company will match donations. House parties and emails are the fund raisers I found easiest and most effective.
There was a tragedy at the race where one of my teammates fell unconscious (there are a couple rumors why but I never found out the truth) during the swim and ended up in a coma. A couple days later she passed away. I don’t blame TNT for this, rather it’s a risk all of us take when we charge out for the swim.
I’ve made a lot of great friends through this program and still train with them. There are a lot of places that offer training programs and coaching but with this one you can do something good at the same time.

I did TNT in San Diego before my first OLY tri (which was supposed to be Wildflower, but I got injured and did a Marathon with TNT instead)
One of the best groups of people I know and 3 years later and another coast away, we still stay in touch. I say go for it, got nothing to lose and you help people out at the same time.

I did TnT too here in Portland. There's a lot of time involved, but it was very rewarding and a hell of a lot of fun training with a group.

understatement

I don't know much about the organization, but you should probably know that in some races, the "purple people" get a bit of a bad rap because
a) most of them are beginners and they stand out when they make mistakes and get in the way (Its not fair to generalize like that but it does happen because even though they don't make more mistakes than any other beginners, its easier to remember that it was a TnTer that did almost crashed into you and not the M24 in the blue top and black shorts)
So - I'd make sure that your group or team coach or however that works makes SURE that EVERYONE knows and intends to follow the rules of the sport.
b) I've heard about (from talking to other people at races) that there has been the occasional case of a TnTer with an obnoxious 'holier than thou' attitude...

I would search and read some of the discussion about this in the slowtwitch forum (they have quite the anti-TnT sentiment over there and some of it sounds reasonably valid, though a lot of it is just people who have never done it bashing it). As people have said above, there are probably many positives to the experience, but you might be wise to make yourself aware as to what some people say.

TNT is one of the best organizations around for helping people get into an active lifestyle. I have done 2 centuries with the Team, and 1 oly distance tri, Treasure Island. I have been involved off and on with TNT for 3 yrs now and love it. Even though I don't race with them now, I train and race with mostly ex TNT people, I still stay in contact with the local chairperson. It's a great group of people, and a worthy cause. Once you meet your honoree, normally someone in your community battling Leukemia, you won't complain about long training days. My honoree was a 7yr old little boy. Trust me, you have to have a heart of brick to not be moved when he yells for you to race hard, knowing full well that he might never be able to do it himself.
It's true alot of TNT'ers are newbies, and make mistakes on the course, but knowing that ahead of time should make others more alert. I still use my TNT wetsuit and get snide remarks and comments at some races where TNT might not have a presence. I then laugh as I beat the idiots out of the water. For me TNT has been a great experience, and I even got my wife into riding after she saw how much fun a century could be.
Sorry about the rambling, but I think it's a great organization, and over 80% of all the money collected goes towards R&D on new drugs, and helping families who are suffering through the cancer.

Yeah, I've heard the remarks too. It's true that there are a lot of beginners - what better way to get from the couch to the starting line - but there are some seasoned veterans as well. I did my first oly with them but I had about 8 sprints already under my belt - not a pro, but hardly a newbie either. Not real sure why the bad rap, but like was said before, we were all beginners at some point and don't forget TNT brings a lot of $$$$$$ in the way of race fees to the event.