Without writing a novel here, I looked at them in 2008 and my brother just a couple months ago. We both decided to go with new, traditional WHs given the cost diff and minimal savings of tankless. As 4Gas$ mentions, the difference in purchase price is large... ~$500 for a traditional vs. $1-2k for the tankless. There's also a big difference in installation costs (or DIY effort) - lots of plumbing work (water & gas), new dedicated circuit, direct vent flue, etc. for the tankless. Whereas replacing with a new traditional is minimal effort... couple hours or a couple hundred $.
During the summer when my gas bill is pretty much just the WH, I'm at about $30/month. Even if the tankless could save me 50%, the payback is a very long time. I'm single no kids, so you could argue that there may be more saving for a family, but if you're using water more often, you're also minimizing the amount of time hot water is being maintained in the tank too.
I think if I were to build a new house, I'd probably opt for a tankless and have all of the plumbing done up front to accommodate. But it's a lot harder to justify in a retrofit. And even being a DIY'er, as well as having electrician and plumbing friends (free labor), it was still very hard to justify the $.
Lastly, Amazon does sell Rheem tankless and traditional tanks if you DIY. Pretty good prices.