mfalco:
The only BCs I've used are rental sherwood silhouette. They were very comfortable and seemed like they would do the job, and they must be durable if they are used as rentals. Other than if I want integrated weight, is there any reason I should purchase a more expensive BC?
To be honest, if you're happy for the moment, ENJOY IT!!!

It probably won't last, depending on your diving style. BC style and fit is something that really affects buoyancy control, and consequently diving comfort, more than just about any other piece of gear. Typically, as divers get more experience they become more sensitive to small changes in buoyancy control and trim, and that's when the quest for a better BC begins. If you can buy one of these rental BCs cheaply and it will get you through a season or two of diving, I think it's probably a great investment, as after another 50 dives or so you'll have a much better idea of what style of BC would work best for you.
Trying on a BC in the store gives you only the most basic information about it, and in some ways can be very misleading. For example, remember that the weight loading on a BC in and out of the water is totally different, meaning it pulls on your body in very different ways. A common issue that comes up is the tendency for some BCs to ride up your torso, especially on the surface. This is something they would never do standing around in the store.
The best thing by far would be to start with a pool session with several different styles of BC; maybe one of the rental sherwood jackets, an expensive weight integrated jacket, a back inflate BC, and a backplate and wing set up. If you could some how gather together this gear and a few different exposure suits to match whatever water temp you'll be diving in that would be great, and you'd probably answer all your own questions confidently. One thing you'd find is that all the differences are greater with a thick wetsuit on, as you need more weight and it's placement is more critical. Another thing you'd find is that an expensive, feature-laden jacket style BC might perform worse than a stripped down simple one, especially in warm water situations.
Have fun!