I've had the Legends on two different bikes...an '06 SEUC and now on my '11 SERGU. On both bikes, the primary purpose was to get me lower when trying to park the bike, back it up, walk it out of the basement, etc. On the '11, the installation was a bit easier because of the new frame and the new compressor mount Legend has...
They do offer a dash mounted gauge (takes the place of the useless air temp gauge), but only in either white or black face. Having a gauge would make things a bit more convenient, but after a while, you get a "feel" for where it needs to be based on ride quality and the angle of view over the windshield.
Basically, any height where the shock does not bottom out will be comfortable with regards to ride quality, which generally means at least halfway pumped up. The rear tire will not scrape under the rear fender, even all the way lowered, but you don't want to ride it there because it is like riding an old hardtail. The Legend bladder does not expand in diameter when you pump it up, and the boot is made of a Kevlar material, so it's very durable. Personally, I don't find the slow compressor speed to be an issue. The chrome switch can be mounted on either side of the bars, taking the place of either the clutch clamp or the front brake clamp. The quality of the switch is very good, as is the chrome finish. It is possible to run the switch wires internally to the bars, though it's a bit of a PITA. The ability to adjust the ride height and quality on the fly is really nice, particularly when you are two up and loaded with gear.
There are better shocks out there if you don't need the height adjustment feature, but the Legends are excellent for the purposes I use them for. On a scale of 1-10, with the stock HD air shocks being a 1 (they are turds), and a shock like the Bitubo being a 10, I'd rate the Legend ride quality at about 7 or 8. IMO, they are the best solution for those riders who have issues getting their feet firmly on the ground when needed. I have no personal experience with the Arnott system, so can't really comment on it.