Nice work on this app. I just downloaded rev. 8.2. Running 10.7.3 on a late 2010 15" MBP. Functionally the app works great, but I've got a couple of suggestions.
Normally I don't take the time to write epics novels like this, but it seems that you're genuinely interested in what we've got to say and are taking some suggestions to heart. So I thought I'd spill my guts. Sorry.
1) Give us the option to use C or F.
In the set target temperature dialog, I think it would be better if we had some more options:
2a) Instead of a strict target temperature, we should be allowed to set minimums and maximums.
2b) Instead of only setting the target for the CPU, we should set targets for the GPU and enclosure.
3) You could have a slider-style control (by slider I mean the kind of element you have for choosing the target temperature) to let us choose whether we want more continuous or less continuous adjustment of fan speeds.
4) Simplify your menubar display. Or give us some different templates.
Here's my rationale for these suggestions:
1) Celsius and Fahrenheit. Normally I'm all about metric, but as a user of smcFanControl, I switched my display to F for the past 2 weeks and only recently got used to it. I wanted to compare the side-by-side temp readings but had to change my smcFanControl settings to do that. This made me think that for people that are more standard-minded, a unit toggle couldn't hurt. I don't think anyone routinely uses Kelvin, but you could throw that there as an option too. (iStatPro has a Kelvin setting- it seems enough people use iStatPro that Kelvin could conceivably be used by more than a handful of users.)
2) There seem to be two general reasons why people would like an applet like smcFanControl (the established free temperature control) or UltraFan. Some people think that Apple's preset thresholds are too high and would rather keep their components (and laps) a little cooler in the name of longevity (and comfort). Other people don't like how their fans spin up all of a sudden. They get used the their fan at a fixed speed and the noise fades into the background. So under some workloads, their fans speeds fluctuate and get more annoying. With smcFanControl, I just set my fans to go at 4800rpm minimum and let my temperatures wander. The noise fades into the background and for the most part, I don't get surprised by a sudden increase in fan speed.
If you had temperature ranges available, you address the noise issue. Ie: target 60C, min 58C, max 70C. UltraFan could make *very* gradual changes to fan speed within the 58-70C range so that we don't hear sudden changes in fan speed. As I type this long-winded reply, I've noticed my fan spin up and down a few times and have become irritated by this aspect of UltraFan. I've read (in other forums) that some people use their MBPs in a recording studio type setting. I'll admit that I don't know too much about audio recording, but I would think that it's easier to "clean up" fan noise from a recording if the fan noise is relatively constant.
Secondly, if you allow us to set target temperatures for the CPU, GPU, and enclosure, we would be able to tailor UltraFan to our preferences. When I'm lounging around with my MBP on my lap, I don't care where my CPU or GPU temps are. I care most about how hot my lap is. iStatPro shows 3 different sensors on my MBP for the enclosure, so you'd have to figure out which sensors are associated with the location of the CPU and GPU. (I'd be welcome to beta test this). Anyway, the point is, sometimes the enclosure temperature matters to a user far more than the CPU/GPU temperatures. This would add a fair amount of complexity to your settings panel though. Now you would need to also allow us to prioritize which temperature trumps which. (Ie: You could hit a CPU minimum of 70C before you hit an enclosure maximum of 45C. So your crotch would be on fire but your CPU is happy as a clam. End result: mad user.)
3) Option for gradual vs rapid fan speed change. This partly is related to the "my fans keep spinning up and down and it's more annoying than if they were just on all the time" crowd. This kind of setting would tell UltraFan whether to change fan RPMs in increments of 500rpm or something closer to 100rpm, and whether these adjustments will happen every 1 second or 20 seconds. (Ie: Min CPU 58C, target 60C, max 70C. 100rpm increments every 20 seconds might be enough to keep me between 58-70C most of the time without me noticing my fan speeds changing.)
4) Tweak the menubar display. I don't personally care for 2-decimal point precision in temperatures. But I do like knowing how fast my fans are running. Someone else might not care how fast their fans are running. So you can either give us a bunch of options or take them all away. Download smcFanControl and have a look at their various layouts. Something like that would be a start. But you could improve on it.
For the record, I use smcFanControl's multiline display with temperature and fan speed. Here's how I'd make it better. Line 1: Current temp rounded to nearest degree. Black font if at target. Blue font if under target. Red font if over target. Various shades of Blue-Black-Red to correspond with how far away from target. Line 2: Fan speed of fastest fan, in black font. Similar color changes to tell us whether the fan will be going faster, slower, or the same. Or, to be kind of awesome, italicize it if it's going to go faster and left tilt italicize for slower.