We just sold our Defiant-Encore 2550. Like others have said, it does have some nice features. At 1.3 grams per hour, it remains one of the most efficient woodstoves out there. The top loading is nice, as is the ash drawer and grate for ease of ash removal. It also lights easily, especially if you crack open the ash drawer door (but don't forget to close it!!). We never really felt like the thermostatic control worked all that well, it certainly wasn't the "set it and forget it" like the brochures claimed. They are a gorgeous stove, and because of the andirons and the height of the firebox they'll take a pretty nice load of wood for the night. Still, we never were really able to get an 8 hour burn (6-6.5 hours more like it....) and come morning between the grate/ash drawer and the fact that there's no firebrick inside the stove to insulate the firebox, we rarely, if ever, had enough coals left to re-start without re-lighting too.
The D/E 2550 also seemed very finicky--I felt like we had to constantly monitor and control it to keep it from over-firing. If we weren't careful and actively monitoring the stove all the time, it would get up to 800 degrees no problem. Over the long haul, that's not good for the stove. Also, as others have said the replacement parts aren't cheap, and, these stoves seem to need somewhat constant maintenance. We got ours for cheap from a family friend, and then replaced the lower fireback, andirons, door and griddle gaskets, and the combustor. Just those items cost us $400 in parts, and had we needed to replace anything else we could have easily spent another $400.
So, if you're thinking about buying it I'd do what others have said. Check the combustor to see if there are any cracks or missing pieces. Pop the two wedges out and remove the lower fireback and get a good look at the refractory box. If it's badly deteriorated, it will need to be replaced and last time we checked they were going for $375. Check where the bypass damper closes up against the upper fireback and make sure that neither piece is warped. If they are, they'll likely need replacement. If it's in good condition, I'd say $650 is a good price. Lastly, the general consensus seems to be that the pre-1994 Defiant/Encores were the best quality, as the mid-late 90's models seemed to have quality control problems.
In the end, we just decided that it was too finicky and too expensive to maintain. They are gorgeous stoves, but we use a stove as a heater, not a piece of furniture. We like our steel Earthstove, simple, robust, efficient, and easy to use and maintain.
Good luck!
NP