Which utility are you using to treat it as a second screen? I've had modest luck with duet.
I'm using Duet too.
It's a little quirky - the most annoying bugs are unpredictable disconnections and occasionally screen freezes, this seems to happen mostly when my macbook locks and reopens. But these things don't happen regularly enough that it causes a major problem for me. It's a bit annoying but I can deal with it.
One thing to note for anyone considering it - it performs really well at displaying text. Things like tailing logs, or displaying docs. However, anything involving typing and using the mouse is a bit painful compared to your main screen because the lag can be tedious and uncomfortable for fast interactions. In addition, I keep the refresh rate at 30Hz when on battery to save power, which compounds this issue.
I've seen those issues as well. I it Duet more as a second display more on my win8.1 laptop, and have very strange 'disconnects' - the iPad screen just randomly goes berserk and technicolor, and I have to quit the app and restart to get it picked up. This usually happens around the same times iTunes tells me there's an update, so guessing it's related.
I currently use an ipad 4, with its 10" screen, as a second screen for my mpb retina 13. I like this setup because it's thin and relatively light so slots easily into my bag along with my macbook, and doesn't require a power socket to run, just USB. I also use the ipad for reading and a few other tasks so carrying it around has intrinsic value in addition to being a second screen.
The ipad 4 does a reasonable job, but it's a bit on the small side. This new pro form factor is, however, perfect - 12.9" and higher res than my mbp retina screen means it really can be a true second screen, with roughly identical capabilities to my main screen. I'm pretty happy about this.
Won't be upgrading just yet on account of the price, but when it drops enough, replacing my current ipad with this will be a no-brainer.