I've used a few of the most noteworthy drum VSTis out there, including Modern Drummer from NI, Addictive Drums 1 and 2, and Steven Slate Drums 5 and 5.5. All have their strengths and weaknesses, but at some point I decided to go whole hog and step up to what everyone seems to agree is the Big Daddy of drum software, Superior Drummer 3.
It's hard to think of anything to say that hasn't already been said. SD3's sounds are huge, detailed, and exceptionally captured. Its mixing capabilities are virtually limitless and endlessly customizable, and its included effects are excellent. It's very intuitive in many respects, but the enormity of everything it can do does mean there is a bit of a learning curve at the start. This passes quickly as you come to understand the thinking behind some of the design decisions and become familiar with the routing (at least that was the trickiest part for me).
What you get out of the software really depends on what you put into it. As I said the samples sound amazing, but they're largely unprocessed so if you don't know how to mix a drum set to make it compete within the context of other instruments in either a live or recorded situation, it's really only going to sound good on its own. Toontrack does have many very informative and helpful videos on their website that demonstrate how to get a raw drum sound more up to snuff using the onboard effects and mixing and routing capabilities, so there is a treasure trove of great information to be found there.
I could go on and on about SD3 but there are plenty of other reviews that do that, and tutorials and whatnot all over the internet. And here I'm so lazy I haven't even talked about the MIDI and songwriting capabilities, but cut me some slack, I have no use for that part of the software. I'm a drummer, I record the MIDI from my performances on my e-drums then use that MIDI to drive SD3. My songwriting is still all out in the meatspace. But I'm sure the MIDI and songwriting stuff is great and all that. :)
So even though there are significant parts I don't use, SD3 is still the current leader in virtual drum software, and its capabilities and flexibility provide so many options there's very little you'd want to do with virtual drums that it can't handle in one way or another. And its expansion libraries are amazing, and the fact that you can run all the EZ Drummer libraries in it as well, all of that opens so many doors that it deserves its own review. Suffice to say that the expansion libraries are just as good or better than the core library the software comes with, and they just keep getting better and better (and bigger, you might want to invest in a separate SSD just for your drum sample libraries, that's what I did and I only have one expansion and the Core library!).
In conclusion, SD3 is the big daddy on the block and has been for a few years now. If somebody was going to knock it off its throne they would have to have some seriously amazing drum software, which I think is why it has remained unchallenged for some time now. It does everything really well and has no glaring weaknesses, and it's enjoyable to work with even though it is a seriously powerful tool.