I've been digging into Geist for a few months now, and it's become hands down my primary tool for adding rhythm to my productions.
One liner: It's a kick-@$$ tool with intuitive control for several different rhythm creation workflows, a top 10 plugin to have in your studio.
Cross-DAW rhythm sequencer (get comfortable, you've got a common playground that works with almost any DAW).
Easily layer your drums (quickly add and mix loops or combine them with your own sample creations with multiple engines).
Customize almost any existing loop to fit your needs (love that loop but want to mix it up? Top of the list on Geist's resume).
Automatic & flexible transients detection (let Geist do most of the work, easily adjust transient points and categories).
Save your creations to reuse (save your kits & layers, patterns, the whole engine, even export as MIDI).
Relatively small instance CPU/Memory footprint (need more pads for that loop and all 8 engines busy? Add an instance and load your current work).
Takes some time and reading the manual to figure out how Geist implements some functionality.
Not supported in Reason, or any DAW that doesn't support a common plugin format (VST [32|64], AU[32|64], RTAS, AAX).
Transient detection and categorization limitations with complex loops, though nothing preventative.
Not all DAWs support (or can easily use) Geist's multi-channel audio routing capabilities.
Not all DAWs support (or can easily use) Geist's multi-trigger MIDI control for an instance loaded into a single track.
I'll add that Geist's multi-level FX processing is another great feature if you want consistent behavior across DAWs, though I often use familiar plugins for that work instead so didn't add it as a pro in my case.
I use Renoise as my primary DAW, and have certainly had to learn how to do a few things - like take advantage of instrument aliasing to simultaneously trigger multiple patterns across multiple engines using MIDI for a single instance of Geist, or figure out how to use Renoise's instrument Audio Routing feature to route different Geist outputs to individual tracks for individual processing by external plugins. I've had to do similar things in Reaper or other DAWs. These aren't complaints, just mentioning them - most of the default behavior in Geist is relatively simple, but capable of much more on demand.
I love this thing. The most important aspect of Geist is how fun it makes the proess of making music.
The sounds, of course depend on what you put in, but the on board effects are varied and sound great. It really allows forensic manipulations of every aspect of sounds at layer, pad, engine and project level.
FXPansion have provided plenty of tutorial videos to get you started and the included sounds from people like Goldbaby are top notch.
It may look a little pricey, but you can do so much with this thing, and if you shop around for a boxed version (as I did) you can pick it up for £149.