I've already decided that my laptop will be my "sound engine" for use with the Zendrum, at least until I can afford a good hardware module. I'm reformatting it, tweaking it to WinXP "stripped-down perfection" (at least as close as you can come), and upgrading RAM to 2GB. I have an Echo Indigo PCMCIA card (ASIO) that works really well and has super-low latency.
So that leaves software. I have Kontakt 2 and Reason 2 (NNXT) as far as samplers go...but I'm considering picking up Battery 3. I think using Kontakt or NNXT just for Zendrum kits is a little overkill...plus I like the Battery 3 interface---it's easier and faster to use just for programming drum/percussion kits.
I've certainly considered BFD and others, but my decision to go with Battery 3 is based mainly on cost---Nova Musik has it for $160...and I'm on a TIGHT budget for a while! So Battery 3 appears to be the best thing going in terms of value. (Features versus cost.) I'm not even that interested in the sample sets that come with it (or any other virtual instruments) because I have thousands of samples I've collected over the years, and enjoy assembling my own sample kits from scratch.
If anyone else here is using Battery 3 (or v2) with the Zendrum, I'd love to hear about it. Is it working well for you? Any particular likes/dislikes? Any "gotchas" to be aware of?