I've been using Battery 3 for some time now so I thought I would post a run down of my thoughts.
I feel this is probably one of the most under rated pieces of software out there today. It's more "tweaker" oriented than most other drum software packages I know of. There's plenty of great preset kits to use but you will have to do some setting up to get them working good for you. It's not really a "plug and play" sort of package but I don't want to scare anyone off either.
Where Battery 3 really shines is that you can load your own samples for a truly open ended software package. It's a drum sampler.
Battery 3 is basically broken into three levels. Samples, Cells and kits.
You can have up to 127 samples in one Cell. That means 127 snare hits from soft to loud that will make up one Cell. Each of those samples have a MIDI velocity from 0-127 (soft to loud or however you want them). Then you assign the Cell a MIDI note number to trigger the sound. Your velocity playing will determine which sound is triggered. The velocity crossfades are totally adjustable. This is the "tweaking" part I mentioned above. You can use an existing "velocity map" from a preset Cell then replace it with your own samples. However, when I tried this I still needed to adjust the velocity map a little. Different samples act differently and needed to be adjusted to my playing dynamics. The benefit is a kit that responds to my playing. The overall volume and pan for each cell can be set too.
Once your cells are created, you make up a kit with them. Up to 127 cells in a kit.
You can then do things with the cells like alternating or cycling through. For example, one cell could be a left hand hit and the other cell a right hand hit. Both cells would be assigned the same MIDI note. When you trigger the MIDI note, the two cells will switch back and forth. This can give you a slight variation in sound similar to playing a drum with both hands. Or you can cycle through a number of cells for many different sound variations or different sounds all together. Many possibilities.
There are also effects you can add to each cell independantly. Lo fi, EQ and compression. There are also Master effects that effect the kit globally like reverb. I would have liked the reverb to be on the cell level rather than at the master or kit level. Still the effects are very good.
My favorite part is still the ability to load your own sound samples to make up your kit. There is an onboard waveform editor in Battery 3.
You can also load loops into Battery 3 and play them back. You can also trigger them as single shot loops or repeatable loops that latch. Hit your trigger once and the loop starts, hit the same trigger again and it stops.
Another good feature is the sounds are loaded into RAM so latency and a sluggish CPU hog program is at a minimum. Still I would recommend a dedicated computer as a drum module in any case.
Hopefully this is a good starting point for anyone interested in this software. I highly recommend it.