VSL Timpani is nice. I have not had the opportunity to check out the new True Strike Percussion, so I am not even sure what kind of Timpani coverage is there.
Donnie Christian's Ultimate Timpani collection is very, very nice--skin heads are mellower in the lower dynamics, and "splattier" in the loud ranges, and you don't have to worry about the tuning!! They're nice Hinger drums, they image well, and there's a lot of material. Probably more coverage than the full orchestra packages, at least a bit more than VSL, although there are some aspects of the VSL timpani that I definitely prefer (more intimate recording, more flexibility of imaging on the back end).
Those are the ones I am familiar with. I'm sure people familiar with these and others can help round out the discussion.
I personally love the sound and versatility of EWQLSO. I use the gold edition but even the silver's timp is great sounding and of course Platinum I claim actually sounds better than the real thing.
Truestrike is awesome sounding too. I've heard the demos and they give me a rush. I'll probably pick those up just as a back up.
VSL for personal reasons I'm not a fan of. Close mic only, no ambiance and too European sounding. Though I would use this library (if I could afford it) for record work, since the big symphonic sound isn't that popular in records, at least not now. Hopefully, if I do more records this could change.
Also, don't overlook some of the older libraries if you're on a budget. I really like the Miroslav timp as well as Siedlaceck( i never could remember how to spell that guy's name.)
And, the GPO samples sound pretty good too. On listen to that library once I found that the timp and the trombones were the most realistic of that library.
Although I am yet to use the True Strike library. I am staring at the DVD's/Manual while I wait to order my copy of GS3 this week. (Christmas special of True Strike was too good to pass up).
Info that you may not know about True Strike: (Pro Edition)
"A set of 4 'Adams' timpani played with Kato medium and hard timpani mallets." Articulations: Normal Hits, Dampened Hits, Tremolo, Tremolo Dynamics, Pedal Glissandi, Effects.
Timpani alone is 5 Gigs and uses 4 mic positions. Close, Stage, Far, and the 4th one is:"The Extra 'Mixed' microphone set is a premix of the close and stage microphone sets.
I also own VSL Opus 1 and personally the timpani is to dry and weak to be really bashed out. Fine for backgroud timpani work. But not exactly in your face the way timpani is ment to be heard . I also still use alot of the free G-Town samples. They are awesome to layer with.
I hope that helps in some way. My personal choice is aways Project Sam as I think their products are awesome. But there are quite a few libraries out there that would suit you. It really depends if you need other instruments. If so, go for EWQL or SISS as both are great products. If you are only looking for percussion I think the True Strike series will be the way to go.