I’ll second the 150mm love. I’ll third it, too… I swear by my 150s. Though they do get in my way at times when I’m really trying to spin fast, they make a good cruising speed so much easier (than 125s) on my legs that I can go maybe 90% as fast for four times the distance. It’s a deal for me.
That said, I have given my 125s a try, and have maybe 50 miles of road riding with them. 20 of those miles were the fastest road miles I’ve ever put in on my uni, and the other 30 were painful and kind of miserable. Whadd’ya know. :=P The 125s weren’t too hard to get used to, and they were decently manageable, and allowed me to get a whole lot more RPM, to the point where 21-22 mph was sustainable for medium-long periods (like, longer than 20 minutes, probably less than an hour). That said, as soon as the wind kicked up, my pace went way down (to around 16mph) and it was HARD to push myself through the air. On 150s, it takes a LOT of wind to punch me under 17mph, while in good wind, 19-21mph is the fastest I usually spin, and neither speed really hurts, where as both speeds on the 125s were pretty tough, good wind or not. So… 125 is too short for me for now. I’ve had a taste of what shorter cranks feel like, and in the ideal situation with no wind and flat ground, I’m faster, and die sooner. It’s awesome to go so fast, but it’s not worth the quick bonk. Besides, the solution is not short cranks; the solution is higher gears. Riding on them, I’m definitely faster, but my spin feels ludicrously tiny, to the point where I think “what the heck am I doing on these?!?” I’d put up with ludicrously tiny if it made me faster in all cases, but any time I need to muscle the thing, it’s leg death or first gear, and I bonk after 20 miles instead of 50 or 60. If I’m in any kind of race with hills, even one like Unicon’s marathon, it’s 150s. If I’m going around a wooden track for one hour, and only one hour, then I think I’d go 125. I don’t know how the German Speeders pushed their gears with those tiny cranks; I have new respect for them after trying that out myself. I really run out of energy a lot faster.
I might try 137s, but I’m afraid to take my 150/125 moments off, because every time I put them on and tighten them, it seems that they press further into the splines. I have less than a millimeter of clearance between the cranks and the bearing holders now.