No one has answered yet so I'll throw in my 2 cents.
I love my Rogers and Spencer, it's definitely one of my favorite cap and ball pistols. It's really at the top in terms of functionality and features in the cap and ball design evolution.
Pros: The design of the hammer and scalloping for spent caps is top notch. You don't get cap jams with an R&S. The loading lever design is really smart and works great as well. They are great shooters.
Cons: Feels heavy but is actually right around the same weight as similiar era revolvers. Some people don't like the shape and feel of the grip. I, on the other hand, do like it.
I'd say $300 for one in very good shape is a touch high. Unless it's pristine or brand new I would offer less. Then again it depends on how bad you want it as there doesn't seem to be lots of them around. Pedersoli is now building them since they took over Euroarms but I'm thinking the price is going to be way up there. I got mine used for much less. I would definitely replace the nipples though if the one you are buying has stock nipples. I'm just one who always replaces the factory nipples with better fitting ones. I would certainly recommend you pick one up as I think you will find they are excellent revolvers. Like I said, I love mine, and I'm a total Colt fan for all it's worth but the R&S is just excellent all around. Once you start shooting it it will completely win you over.