I'm in the same boat as you, but I don't tend to voice my opinions on Test::Unit because there is such a strong opinion for RSpec and Cucumber. IMHO, I tend to like my tools to be tried and true, and not do any fancy magic.
Learned my lesson the hard way(s) with Rspec and Cucumber a while ago - total waste of my time.
There's a group of devs that have fooled themselves into thinking anyone outside their group understands how they are testing. I've seen this firsthand. DHH has always been right, imo, in this regard; test what you think is important, use simple tools.
TestUnit still serves me well and it perfectly fits my needs of do more with less.
I used Test::Unit for a long time, but I found it to be a poor tool when doing integration tests. RSpec lets me do both within the same DSL quite easily. That's probably why a lot of people flock to RSpec. I still have old Test::Unit code sitting in my RSpec suite that I haven't moved over to RSpec's DSL.
That being said, to a developer writing tests and doing TDD, Cucumber is a speed bump that doesn't need to be there. RSpec is great without lumping more crap on top of it.
