How does that thing affect your power bill, considering we just had the hottest June ever?
I had my highest electricity bill ever this June.

But I can't say for sure it's all from the garage, I got a kegerator (in the house) and the wife's home all day with the baby so the AC is running more, too and the TV is on all day. Also, I just finally finished the attic insulation (R25) during mid-June as well as the garage door and that should help quite a bit. I usually run it Thursday or Friday night through Sunday night, leave it off on the weekdays.
What size unit did you get, and what size is your garage? I'm thinking about an 18k btu unit for a 600 sq ft fully insulated garage (in Houston).
I got the 17K btu unit, Mitsubishi, and my garage is 400 sqft. I would tell you that I would go even bigger if I was to do it again and here's why. If you leave all the doors closed then this 17k unit does great, cools it to ~70 something on a hot day in my little garage. But the problem is if you get to welding, cutting, grinding etc and you get fumes in there. I open the doors periodically to ventilate and then it's tough to get it cool again.
I read all this stuff about how it won't dehumidify correctly if you have too big of a unit, and did some calculators to see what size I needed (and felt I got a slightly oversized unit). But I think those are assuming that you run it all the time with the doors closed, in a house type setting.
Admittedly, I have a strong tendency to over-engineer stuff, but I'd sure consider going bigger with your mini-split for a 600 sqft garage, based on my experience, if you can fit it in the budget.