What server-side Bitcoin clients are available to serve information? I have been toying around with the idea of building a bitcoin app. My research has thus far led me to conclude that it makes sense to run a bitcoin client daemon on my server in order to provide an API for my mobile app.

Is bitcoind any good for this? I used the -server mode of the Bitcoin-qt client for mac, as explained here, and it seems to be very slow for most of the RPC calls. Is the daemon any faster? It seems like something this slow is not preferable to run on the server, and would almost force me to cache some responses that would probably be unnecessary for a faster running client (obviously, down the road cacheing would make sense, but for the initial small scale of this, I would like to avoid it.

Are there better server-side clients I could run that would perform better than bitcoind? Do any of them provide better interfaces that JSON-RPC, or is there some reason that JSON-RPC is intrinsically better for this sort of use-case? How about a REST API?
It depends on what information you require. Bitcoind can provide you with information through its API, but you'd need to be running it yourself and it only works through HTTP JSON RPC. There are other sources of similar information, like blockchain.info's API, which can also handle other formats.