Cutting muscles for knee surgery is old school. The muscles are usually stretched not cut, but I suppose you should ask anyway.
I had both of mine done in 2008, 9 weeks apart. Best decision I ever made. Rehab or a good exercise program before surgery would be helpful to build up muscle strength. I was going to Curves and that helped a lot. Key to recovery is to do the exercises faithfully.
Check out
Hip & Knee Joint Replacement Info | Forum | Clinics | Surgeons | BoneSmart for lots of info about both knee and hip surgery. There are forums there where you can get real time answers to your questions from folks who are contemplating or recovering from surgery. It was a lifesaver for me to be able to talk with people who knew exactly what I had been through.
Good luck and don't hesitate. I can just about do anything I want WITHOUT pain.