Beta 1 - pretty good overall, but horrible battery drain.
Beta 2 - pretty much beta 1 but with much better battery life. This one was my favorite (looks-wise).
Beta 3 - introduced the new thicker font (which I hated), randomly froze when left idle (requiring a hard boot), randomly missed phone calls with no indication that the call ever came through (unless they left a voicemail), notifications were hit-or-miss as to whether or not they'd show up in the NC, Settings crashed any time you tried to select a new wallpaper... This one was by far the worst beta ever. I reverted back to beta 2 after only a day and a half.
Beta 4 - fixed everything wrong with beta 3 except the notification issue. I've now accepted that the thicker font is unfortunately here to stay.
Beta 5 - seems to have fixed the notification issue from beta 4 for the most part. Sometimes I find that I miss text message notifications completely, and only notice them due to the app badge, but this isn't very common or consistent. Overall I'm pretty happy with it.
I just wish they'd change the font back to the Light font of beta 1 & 2, or at least give the option to. The current one just feels too big.