Suppressor sales are booming, and with more and more Class 3 (SOT) dealers around, it's easy to handle the product you're thinking about buying, or even get an in-person shooting demonstration. The Form 4 process is not complicated, just time consuming (like gcrookston said), but the Class 3 dealer will basically help you through it. The most annoying thing I found was getting the passport photo and stopping by the Sheriff's office.

A suppressed AR is cool, no doubt. At the 3Gun match yesterday Ray shot our 7" suppressed M16 (and used full-auto on stage 1), and it sounds really good. However, there are some downsides of a suppressed AR: not as quiet as a bolt gun, increased heat and backpressure, rifle gets dirty a lot faster.

In terms of "bang for buck", I think a .22LR suppressor is the most fun for the money, while a suppressor on a bolt gun works the best and provides the most real benefits to shooting.

As far as the ear protection issue-- if you are serious about retaining your hearing into old age, when shooting full-power centerfire ammunition, you should double up, e.g.: muffs and plugs, or suppressor and plugs. If you are in a situation where you can't wear plugs (such as hunting or as a LEO), then a suppressor will reduce the amount of hearing damage you accumulate over time.