I've got a 580 series and I couldn't be more pleased with it. Read here for the preliminary review with accuracy notes & pics:
http://sargesrollcall.blogspot.com/2008/10/mini-14-580-series-ranch-rifle-last.html
Since that article I have put at another 350+ rounds downrange, most of which were reloads I put up in mixed military brass using 55 grain bullets and H4895 powder. Practically all of it was fed from 20 round ProMags and all factory ammo of every bullet type, and every in-spec reload (plus many which weren't in-spec) have fed flawlessly. I've trained several LE folks on various 'M4gery's' and malfs with factory ammo are not unheard of with them; they can be downright picky about reloads. I haven't seen nearly as many Mini's but the ones I have seen worked. For decades however, rack-grade AR's enjoyed a substantial accuracy advantage because it was a rare pre-580 that would really shoot.
I am something of an accuracy buff and I won't keep a rifle around if it won't shoot. What I am not is a 'barrel burner' and I don't sling ammo away just to make noise. The vast majority of my current Mini's round count has been fired carefully, at 200 yards. As time has progressed, I'm convinced it is shooting better than when I first evaluated it. The little rifle loves Sierra's match 55 grain HP's and produces occasional 200 yard groups right at two inches. It's no slouch with 55 grain Winchester bulk softpoints, which like the Sierra's are loaded exclusively over H4895. I had a cheap 3x9 scope on it and even with the barrel warm it would explode water-filled Coke cans at 200 yards, one after another or until
you flubbed a shot. I haven't shot many AR's or M4's at that distance but judging from 100 yard work getting their sights zeroed, this Ruger will run with any rack-grade AR I have shot. I can't promise you they are all this good; I can promise you that this one isn't for sale.
The AR enjoys a substantial advantage over the Mini in availability of magazines, accessories and aftermarket parts. The AR has much better magazine locking system. The ease and speed with which magazine changes can be made, would make the AR the sensible choice for any protracted firefight or 'Run and Gun' competition. For most any other use, 20 rounds is plenty and I have no qualms about using the Mini as a patrol carbine.
The Mini is also hard on brass, throwing it 18-20 feet and occasionally mangling it in the process. Differences between the typical stock trigger pulls of both rifles are inconsequential- no compliment to either of them. Both may be improved substantially and you do at least get a 2-stage pull with a Mini.. The ergonomics of the Mini simply work better for me and I favor blue steel and walnut (or a reasonable facsimile thereof) on my rifles. I think once I get some decent glass on this gun and exorcise that damn lawyer from the trigger, this Mini will flat shoot. I'm also going to swap out the gas port bushing so I don't have to chase brass into another zip code.
If you're going to buy a Mini and accuracy matters to you, get at least a 580 series.