I've done it, back in 2009. Obviously integrated shifters are much better, but it's still mostly about racing smarts and legs. I found it wasn't as big a problem as you'd expect, partly because I was on a 6-speed cassette, so it's not like you're constantly reaching down to shift. But you have to deal with being in the wrong gear semi-frequently. Biggest thing I had to learn was to downshift before entering a corner, rather than on the way out. That's a better habit anyway, and it stuck.