Just finished priming my whole upstairs with one of these.
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My renters left it in the garage when they moved out for some reason. I don't think I would have ever thought to buy one (they're around $200) but since it was here I decided to give it a shot. Had some trouble getting it going and had to replace the nozzle and the gun filter, but holy shit does this thing save a lot of time vs. rolling. You have to do a lot of masking and ti takes half an hour to clean out when you're finished but being able to paint a whole bedroom in 15 minutes makes it all very worth it. I'm at least going to use it to do my ceilings. I'm not sure about the walls. I've read that you won't get a nice enough result to use it on interior walls vs. rolling. I don't know though. I've painted some stuff with a roller that didn't exactly come out flawless. I had some problems with getting too much paint in spots in the beginning but the more I do it the less of a problem that is, and I just went back with a mini-roller and smoothed out any spots that ran or dripped once I was done spraying.
EDIT: Oh and another tool I just became aware of is a 3M hand masker.
This thing is fucking awesome. I had seen them before and didn't really think that they would work that great but it works perfectly. Way better than I would have expected. Lets you mask things very efficiently with just one person and if you've ever tried to tape up plastic alone you know how frustrating that is. Works with either plastic or paper. Well worth the $30 if you have a significant amount of masking to do.