I'll tell ya what I know. I have constant muscle spasms, that I'm told are due to scar tissue enveloping the sciatic nerve since my surgeries. I've had two now. First a Hemi-Lami/Micro-D in 2004 and then a PLIF 2 Level Fusion March 9th 2007. Since the PLIF the muscle spasms have not stopped EVER, and I've been on muscle relaxers since my surgery, so over a year now.
At first I was taking Zanaflex 4mg, but about 3 months ago, PM doc changed to Robaxin 750mg. It has really given better relief. The spasams are much worse at night and wake me out of a dead sleep. The Robaxin seems to be a bit stronger, and allows me to sleep through the night.
I no longer take Ambien or Lunesta (both are sleep aids) which I had taken early on after my surgery due to not being able to sleep b/c of the spasams.
I've not had any problems with the Robaxin, and though it's Rx to be taken twice a day, I generally only take it at night because it does tend to cause some drowsiness, so I don't want to take it when I have a long day of teaching. If it's a short day, and the spasams are bad, I might take it mid-day when I get home. It just depends.
So there's my story, and I'm sticking to it!
Don't know if I helped you at all or not!! Not really sure about any real long term effects of using it, and never really asked the PM. Just figured he wouldn't prescribe it long term if I wasn't supposed to take it long term! I dunno though!