I had a UV777BK for about um, a year and a half or so before I got my EB/MM JP7.
I no longer own the UV777BK. It was a nice guitar but honestly paled in comparison as far as playability, quality and fit and finish.
After I recieved the EB/MM I never played the UV again so as a result I sold it about 6 months ago to a friend.
Given the choice for me personally, the sound I like, the tone I like and the way I like a guitar to play, I'd buy the EB/MM again in a heartbeat.
As a side note, I found the pickups in the UV777BK to be stale and lifeless. I gutted it right after I got it and put in a Tone Zone 7 and PAF7; huge improvement. The EB/MM still has the stock pickups, I've decided I like them and have kept them.
If you are a whammy-bar king (I am not, I'd have gotten it w/out if it were an option) you may want a double locker I guess. I don't have any tuning problems even if I use the bar, but I don't use it often. As a result it's certainly nice to have easy string changes.
I really like having the piezo as well, it's probably the best sound piezo I've heard yet in a solid body. It sounds infinitely better than the LR Bagg's setup in my PRS Archtop.
The UV777BK is a fine guitar, just for me it didn't feel like the quality or caliber of instrument I prefer so I got rid of it. I'd never get rid of the EB.
But that's me and my tastes, your mileage may vary.