All I can say is that RSpec makes testing your Rails app even easier. Seeing as how Rails came with by far the best built-in testing framework I've ever seen, that's saying a lot.
If you've been using Test::Unit tests for a while, and you've built up a large suite, then you have a tough choice to make. Migrating to RSpec will require you to rewrite a lot of code.
However, if you're starting out new, get RSpec now and don't look back.