For US users snapswap is better and easier for deposit and withdrawal.
Also, Bitstamp charges percentage fees for ripple internal transfer. Snapswap doesn't.
Ironically, this service may not be suitable for implementation by a US person without applying for money transmission license. Non-US person not subject to US laws has the advantage to implement it.
You'll need a considerable amount of USD reserve to do this business. Another option is to create an IOU USD asset, say botsusd, just like botscny, and let users trade in the internal market from botsusd to bitusd. Then the service performs as a gateway from ripple IOU USD to bitshares IOU USD.