I installed 5.1, only to notice it's unusable since I use synchronization a lot and there is known bug that it hangs after closing sync (Bug 915).
What gripes me most is that according to change log it's fixed already, just not released. I suggest you release 5.1.1 and start to collecting stuff to 5.1.2, that sync bug is real showstopper for me at least.
I think it's appropriate to release often especially when you just changed major version number, people expect it to be buggy after major change, but they also expect those crash / hang bugs to be fixed fast.
I would not like to install 4.x again just to be able to sync, since I like a lot all of the new features in 5.1. Great work nonetheless.