I've not had one, but I can tell you as a home shopper that the tankless systems are a real up-and-coming trend in new homes.
One other thing to look in to is a Hot Water Recirculation Pump ... They get the hot water to the taps much faster and save a lot of water. My understanding is that they don't work on all water systems, but as long as you are planning to get your tank replaced it might be worth looking in to as a less expensive alternative to a tankless.
It may not save you as much in the long run as a tankless system, but the initial cost would be much less.
For example, Costco.com is selling one for $180:
Costco - Watts Premier Hot Water Recirculation Pump