So I've had time to do a little short of 30 miles of commuting on the new setup. It definitely works better than it did as a ten-speed, five-speed, or its brief period as a 7-speed-but-not-really. The biggest thing I notice is not having to think about shifting. I suppose I didn't have to in the first place, but there's something about a multi-speed bike that makes me feel obliged to use the gears. So if I was coming into an intersection, I'd go down a gear to facilitate accelerating out of it, and then I'd have to shift up again somewhere between the timing line and the next crosswalk. I'd already started doing that a little less with this bike just because the shifting system wasn't very good, but now it's not there at all.
Sometime after I cross the intersection and haven't had to fight with the shifter, it occurs to me that the bike now accelerates much, much faster. I think the biggest improvement to the acceleration is probably just that the derailleur was pretty badly gunked up, so it had to have been generating a lot of friction. But it can't hurt that the whole bike's a little bit lighter now. Certainly it feels a lot lighter in its acceleration and general handling. I notice a more direct feeling when I start pedaling, and I suspect that the lighter feeling of the bike in general is to do with that, although the rear derailleur certainly weighed a lot.
The big disadvantage to the new setup is hills. This bike has never been that good on hills, I think because of the friction generated by the rear derailleur and that it seems to be a fairly heavy bike in general, and it's a lot worse now that I can't give myself some mechanical advantage. The ratio is have is about 5:2 (actually 52 teeth on the chainwheel and 20 on the freewheel, for people who really want to know) which is a pretty high gear, so it doesn't take much resistance before I have to stand up and really push into the pedals. That applies to starting from a dead stop as well - since I geared the bike to perform best cruising at high speed, starting is a bit difficult. What it comes down to is that the bike has two gears: sprinting and soft-pedaling. On a hill, of course, there is no soft-pedaling, which leaves either charging or going into the anaerobic zone. I have to admit that I do run out of steam before I hit the top of the arc crossing the Brooklyn or Manhattan bridge and I don't necessarily have enough uphill sprints in me to ride all the uphills traveling uptown if I hit the lights wrong, but I stand by my gearing choice. With 22 teeth, I sometimes got too fast to pedal smoothly.
So far, I have to say that I really like my commuter as a single-speed. It has a much more aggressive feel and I think my speeds are a lot higher because it's tuned to be best at about 18 mph. The handling's quicker, the bike feels more agile, and it's more fun. Because of the necessity of sprinting more, I suspect that it's also going to make my speeds creep up on my better bikes. I think a single-speed layout's not for everybody - it really requires an athletic style of riding. It reminds me of switching from a soft, intermediate ski to something bigger, stiffer and heavier - this bike really needs to be pushed in order to ride well, but it gives back much more when I do than it did when it had all the components I've thrown out over the year or so that I've owned it.