It's legit. I used a gym bike (not even a nice one) for close to 50% of my miles two years ago during a busy patch in my life and had a top 3 bike split in my AG (M30-35, quite competitive) in the LA triathlon, which is a pretty legit race in terms of competition showing up. The gym bike was one of those ones with a fat seat, wide pedals, and squeaked like heck - had duct tape all over it as it was clearly past its time, but it was the most efficient way for me to train by going from pool to bike or vice versa. It's legit - ride the crap out of it, do intervals often, and you'll be fine. I really punished myself on that stationary bike - the other folks in the gym were horrified at the massive sweat pool and pained look on my face during those intervals. Made riding with competitive roadies not a big deal, even on the big accelerations. (I didn't train with the triathlete cyclists because they were too slow in general.)