I have been testing out Silverstripe this past couple of days and so far I like it. However I seem to be getting Memory Limit errors (when rebuilding the database) and judging from the post in the forums, its a know issue with Silverstripe. I am running it locally (using WAMP). My current hosting is using PHP4 and I am planning to migrate to another server that has PHP5. The only concern that I have is that can Silverstripe run on a shared hosting environment.
It can, though there are some pieces of Silverstripe that seem to have quite high overhead - that diabolical browscap.ini comes to mind. You may need to make a few mods to the Silverstripe bits and pieces if you encounter anything strange like inconsistent "out of memory" errors, etc.
But other than these quirks which may hopefully be ironed out over time, I believe it should all be fine. Just... maybe a little slower than you expect ;)