Another vote for the Sprintech mirror. I've used 'em for two or three years on a couple of drop bar road bikes. No problems. Very clever design, well made, excellent value.
It's a two-piece doodad that uses a ball and socket joint to adjust the mirror. No problems with the joint coming loose. Occasionally I knock the mirror with my knee when standing to pedal, but that's because I chose a 38cm or 40cm bar for that bike. No problem, takes a second to adjust the mirror. Part of the problem isn't so much the knee clearance for the bar end mirror, but the placement of the cables. Between the brifter cables crossing in front of the handlebar, and the internal cable routing, there's a little loop of cable by my left knee that unconsciously nudges me to flare out my left knee when standing to pedal. On a bike with external cable routing that didn't touch my knee, I wouldn't flare out my knee so the narrow handlebar wouldn't be a problem.
The other bike with external cable routing, and 42cm wide drop bar, has enough clearance that I don't knock it with my knee.