I've been using a Minn Kota Terrova i-Pilot (24 volt, 72 lb. thrust) since 2013. It is FANTASTIC!
I often wonder how in the world I ever managed to fish without it. I tell everyone, "It doesn't matter what boat you buy, but if you don't put an i-Pilot Trolling Motor on it you're making a horrible mistake. You can scrimp on the boat, but don't scrimp on the trolling motor."
FYI, I did have to have the steering head replaced last year (by <a href=http://www.swaffordtrollingmotor.com/>Swafford Trolling Motor Repair</a>). HOWEVER, I am a fishing guide. I'm on the water 150 days a year, give or take. Therefore my equipment probably gets used four or five times as much as the average angler. In other words, my 3-year-old i-Pilot had the equivalent use of 10-15 years normal use by the average angler, maybe more. It cost $250 to have the steering head replaced.
The Spot Lock feature is awesome. I use it regularly. However I will tell you that it works best when there is either wind or current for it to work against. In dead calm conditions on the lake, mine sometimes gets flaky (won't hold position as well and sometimes has to correct and it will often over-correct). However, I am told that on the newer models they have enhanced the Spot Lock capabilities to make them more reliable.
The only other issue I have is that, when it is running, my trolling motor interferes with the side-imaging on my Humminbird. It pretty much negates the use my side-imaging when I'm moving with the TM. Works perfectly when moving with outboard. I suspect however that is more of an issue of installation than it is the trolling motor... but it is something you should be cognizant of.
I am lusting after a new Ulterra with the "Auto-Stow/Deploy" feature. I just can't bring myself to spend the bucks when the current TM is still working great!