FusionIO is NOT an "accelerator" product as in "caching", for which there are now numerous products on the market. FusionIO is just a very fast storage device. And yes, if your data can be completely contained in memory faster tlog operations is a win - but so is flushing dirty data pages down to disk, which can be a killer on rotating media for an active OLTP system.

I have been a proponent of FusionIO since WAY before they were a house-hold name, including spending time with their lead driver developer and doing various SQL Server benchmarks, etc. I have quite a few clients that have migrated to them with very good success. BUT BEWARE: I also have numerous clients that have migrated to either FusionIO or other SSD-based storage and their crappy design/code/indexing/maintenance/etc sometimes leads to (massive amounts of) deadlocks because the system is no longer hobbled by poor IO.