I've been using Sia since end of August. It has a nice UI, I think they must have upgraded mostly the back end API for the latest release. It does take awhile to upload files. From personal experience, I do not advise trying to upload a large multi-GB folder (i.e. photos) at once through the UI.

To host storage, it'll cost about $20-25 USD to hold enough coin to host. Of course this is depending upon the exchange rate (currently USD $0.0004/Siacoin).

To purchase storage, it is actually quite affordable as you described. Just watch for the delay (confirmation time I believe) in purchasing and obtaining the storage space, I accidently purchased storage 3 times. All purchases of storage are performed in the native Siacoin.