Geist is one of my most used VST's. I certainly use Battery a lot less since getting it, although Battery is fantastic also - just in different ways. You really are spoiled for choice with Geist when it comes to song and pattern arrangement though. I think I started off composing patterns in Geist and triggering those patterns via MIDI, now I'm really enjoying using the song arranger. It's great to be able to do everything inside the VST, although you really have to keep on top of the sync between Geist and Sonar. By which I mean, if you happen to insert measures into the Sonar project, Geist does not insert the measures at it's end, you have to insert them manually yourself (which is actually no big deal).
But there are two other arrangement techniques Geist facilitates that I have yet to delve into - one is dragging patterns from Geist into a MIDI track in the host, the other is the "scenes" feature. I love the idea of dragging patterns into Sonar, but I'm not sure how it maps the pattern graphs to CC's. I'm sure it's all very well thought out and logical but I've just never attempted it in a project yet. Scenes are a very cool feature - you can save the whole state of Geist (i.e. which patterns are playing across all 8 engines) as multiple scenes and then trigger them via MIDI. I can see that being a boon for live performance, and indeed as part of a project, but I've never got around to incorporating it into my workflow yet. Perhaps in my next project.
I just took advantage of the sale and bought Goldbaby Synthetic, Cinematic Percussion and BFD Remix. A bargain at $54 for all 3. Downloading them now so I haven't played with them, but they all sounded great in the demos. I'm a sucker for new kits and loops. Slicing up loops is my absolute favorite thing to do with Geist. I love how easy it is to edit samples and apply envelopes to them etc. The workflow is just so smooth and intuitive. Yes it has an itty bitty interface but I really don't mind that now.