Installed the developers preview of Android L on my Nexus 5 this morning.
Initial impressions are good - haven't found any apps that are not working yet (with the small exception of a display glitch when trying to post a Facebook comment), and have had no instability or crashes...so far!
Can't say it is a stunning change from 4.4.2 so far - will need to dig a bit deeper and of course not all functionality is implemented yet. That said, for the first time I have actually been using the Google keyboard instead of SwiftKey and it is working well for me.
I am fully expecting a dip in battery life, so will keep an eye on that.