I've put RockShox on a cruiser twice: first time back in 1997 on a Schwinn Cruiser Six and second time last year on my '99 Knuckle. Once you know the steering tube diameter on your frame, it's pretty straightforward and easy to do.
On my SC6, it was a direct swap - both were 1 1/8", threaded, and had same head tube height. I bought the fork + brakes used from a friend who broke his Specialized Rockhopper, and I ran into no snags whatsoever when I put them on my bike. Even the V-brakes (which were relatively exotic at the time) fit on my rear Diacomp brake posts. The end result was a pleasing upgrade!
For my Knuckle aka "OCD", I bought a new threadless RockShox Dart 2 disc brake fork in 1 1/8" from BlueSkyCycling.com as well as a stem for a total of only $105. When they arrived and I went to install, I found that I needed to get a threadless headset (wiki link for diff)
, another $20 from my LBS. If you are switching from threaded to threadless, you must replace the bearing cups, which are included in a threadless headset kit. You must also have a stem for threadless forks.
New threadless suspension forks always come with a very long steering tube so that you can cut it to personally fit your ride. It's an easy swap and worth it, IMHO.