Jamstix

I really really do not know why I did not fork out 99 bucks for this a long time ago. It really is an amazing product. The trick is to set the input trim and velocity curve so that the 'power' knob moves smoothly up and down with your playing dynamics. At that point choose a kit, style and virtual drummer, (each of whom has their own quirks and unique playing style), press play and - voila. Instant drums, which track your playing and vary from quiet stick and ride through to full on dynamics, fills etc as you play louder. It's extremely quick and easy to define song patterns by typing in things like 'VVCV' for verse and chorus, etc; this is a product designed by musicians for musicians, in my opinion.
 
The included kits are fairly decent and have the merit of loading almost instantly, but you can route the output to any drum plugin you prefer.
I don't know if it's possible to somehow have it follow tempo changes other than presumably adding them subsequently but I can live with that. What Jamstix does is take away the painful tedium of fiddling with loops, patterns, grids and trying to tap in drum parts either on the keyboard or via pads. I know lots of people can do this well; I'm not one of them. Highly recommended. (ps: tested with Reaper but I believe there are Sonar users)
 

I have a few templates for various other drum software. People tend not to get past first base (me included originally) and yet with a few simple adjustments it can be quite a powerful tool. I also use it to jam to - it's in the name.

I agree that it's an amazing program and the tech support is very good. I'm just starting to get to grips with it. However the main complaint I have is that the manual is not very well written. You come out with many unanswered questions about how the program works, and some of the current manual pertains to earlier versions - it has not been updated. If I had another complaint it would be that the GUI is a bit 90's video game looking, but that's pretty minor, it does the job. 
 
Recently I've been working at getting it to drive Battery for electronic styles, and although I've heard many people say it's not really geared toward electro genres I would beg to differ, you can get some fantastic results. I agree that it's very quick and easy to get a song up and running with the song structure shorthand, but the key to making great sounding beats that gel with your music IMO is by importing your own basic MIDI grooves and/or playing with groove weights and forced hits in individual bars. 
 
I just wish they would hurry up and restore the ability to drag MIDI clips to the host. This has been disabled in the latest version.