Anyone else use only a bike for cardio?

To make a LONG story short:

My physical therapist recommends that I stop running if I want to continue to be able to play tennis. He says that I need to limit the amount of high-impact concussive exercises that I do on my knee. (Rebuilt with cadaver ligaments after a bike vs, truck accident in 2003.) Basically I can run OR play tennis, and I'd much rather play tennis.

So he recommends an exercise bike. Not spinning - because the high pressure standing up is just as much impact loading on my knee - but just seated cycling. Upping the resistance doesn't seem to bother it as long as I'm sitting.

So can any of you get a good sweat on with the exercise bike? Should I get a heart rate monitor maybe? I just don't "feel" like I'm getting the same level of cardio that I did when I ran. When I ran I was sweating and breathing hard but I can't quite get there on the bike. I'm not sure if this is real or just my perception.

What do you guys think about the exercise bike as a main form of cardio?

First of all I admit I am no where at the level of fitness you are.....my behind is not ready for any kind of bike....but after reading your post I was wondering what about swimming for cardio? You can get an intense workout and you'll be protecting your knees so tennis can still be a huge love in your life....just a thought....unless of course you hate the water....then nevermind