I've had one for a few years and like it a lot. I've always thought it'd be a great truck gun around a ranch, and that seems to have been the idea behind its creation. Mine has never even hiccuped on me-- not a single FTF or FTE. Wood stock, stainless steel, tough little rifle.
As people will tell you, they're not very accurate. 3" at 100 yards is about the best I've gotten out of mine, but that's good enough for just about anything you're likely to actually shoot at. The ridiculous out-of-the-box accuracy of the typical AR has made a lot of people feel that anything over 1 moa isn't good enough for them. You can spend several hundred dollars more on aftermarket parts to tighten up your groups and get a 1-1.5 moa Mini, but by then you're well over the price you'd pay for a basic AR.
With a little range time, you can learn to account for the Mini's tendency to start stringing its shots higher and higher as it warms up, and it really does warm up. That little barrel gets hot! By the 10th round of a 10-rd mag, my shots are going about 6-8" high at 100 yds. Crappy barrel harmonics. Apparently getting the barrel chopped to 16" and recrowned will give it much better harmonics (less whip) and accuracy and, depending on what the gunsmiths are like in your area, can be a pretty cheap fix.
Aftermarket magazines are the biggest issue. Either get the factory ones from Ruger (at least $50 a pop) or go with John Masen. All my mags are John Masen and, like I said, I've never had a single problem. As I recall, they were around 20 bucks each, and he ships fast. I've also heard PMI makes reliable aftermarket mags. Apparently most reliability issues anyone's ever had with the Mini have been because of aftermarket mags.
In short: great little rifle, dead reliable, lots of fun to plink with, don't mind if it gets a little banged up as it's a workhorse not a showpiece. They're so over-engineered (like most Rugers) that I wouldn't hesitate to buy one used, and at that point you're definitely saving money over an AR.
Lots of folks will still tell you "just get an AR," though. Can't go wrong with either, I guess.