Zero. But from those who have it, no one I know or have heard from would say the preview build isn't worthy of daily use, unless you're a dev or don't plan care about bugs and FCs. Maybe for a tablet but not a phone.
It's alpha at best and very incomplete. Lots of UI stuff is missing too. Nothing wrong with playing with it though if you absolutely must. You're better off waiting and running Art right now in the mean time
It's not "very incomplete", and there really isn't a lot of UI stuff missing. It's about as feature-rich as KK, with some things missing (like a way to clear all notifications at once) and several new features. Some apps currently don't work well, but the vast majority do. FCs are very rare. Not 100% of the UI has received the Material design treatment yet (there are still some old Holo menus in there and such), but most of it looks new and fresh (this doesn't apply to most apps, obviously). Everything is smooth and fast as shit.
As I said, I've been using it as a daily driver since it was released, and except for certain apps not working well and some other minor bugs here and there it's been great. The more serous issues some people have been seeing seem to always stem from not fully wiping the phone when flashing the L image. That's on those people for modifying the flashing script provided by Google (it includes a wipe flag, which these people edit out because they don't want to lose their data). If you're gonna try this preview out you've gotta do a clean flash.
I'm just very picky. MMS not working would be bad for me. Plus I'm a performance Nazi. I hate bugs and hitches of any kind, so seeing that would annoy me. Plus the UI isn't done, so seeing a mix of KK and L assets would be jarring, to say the least. But that's just me.
I'm really excited for L. This is the biggest step for Android since ICS, if not bigger
MMS not working properly (although this might be more of a Hangouts issue, as people have reported being successful with other apps) is a bummer, sure. So if that's very important for you, yeah, maybe stick to KK for now.
Performance is one thing you DON'T have to worry about if you give L a go. It's better than KK in almost every way. Animations and scrolling is smoother, hitching/jank is pretty much non-existent, apps open and run faster than ever (although this might just be thanks to ART, which I never tried on KK), etc. Android has never felt closer to a true 60fps experience than it does with L.
Yeah, the random KK-looking assets here and there are unfortunate, but overall the UI looks much better. And it's not really a "mix" of assets, it's just that certain screens haven't been updated yet. And the status bar still looks the same. But it doesn't look like that's gonna change dramatically anyway, just some small icon adjustments and such.