I've been using Oneiric since its release and in many ways it is a giant leap forward from Natty Narwhal. Unity is still horrible mind, but a lot of the rough edges (multiple / external monitor support) have been softened and the overall package is more stable.
Some things still grate, particularly the global menus and the overlay scrollbar, but they are manageable.
A great resource I think to checkout for anyone using Oneiric is webupd8. You can find a lot of tips to bend Oneiric to your will.
For me, that means getting an official JVM installed Installing Oracle Java 7 and getting a sane desktop environment configured with global menu and overlay scrollbars zapped away Things to Tweak After Installing Oneiric.
Linux Mint (now on release 12, Lisa) is increasingly becoming a viable alternative to Ubuntu. I've played around with it a fair bit, but I still think it is not quite there yet in terms of 'just working' when compared with Ubuntu.
For someone that needs to get work done with a minimum of fuss, Ubuntu is still the leader. I do hope though, someone can work out how to marry usability and power in a slightly better way than I feel Canonical are managing at the moment.