I too recently bought a Mini. As to magazines...I'll give you my experiences.
The Ruger factory mags worked flawlessly. They locked up into the well solidly. I've a 5 round and a 20 round.
I have a few 30 round PrecisionMag Ind. units that again worked flawlessly. Locked in well as good as the factory mags. Got them from
http://www.clipsNstuff.com for about $38.00 US. They still has them in stock.
Got a Triple K mag that again functioned flawlessly, but had to fiddle with it to get it to lock in properly. The finish isn't holding up. Also, it dented at the front hole, and the lug at the back is showing signs of serious wear after VERY limited use. It looks as if the rear lug when it was welded in place wasn't aligned too well.
Then I come to the USA mags. I've got 3 of them. IMHO they are crap. Next to impossible to get locked in place quickly. Was getting failures to feed. Both...rounds getting stuck on the way into the chamber-bolt open(bad), as well as the bolt riding over the top of the magazine giving you an empty chamber-bolt closed(even worse). Paid $13.00 US I'm gonna try to work on them and see what I can do. Maybe a stiction problem with the follower/spring.
Just for some information...my Mini is a 181 series which I got from a friend. Rifle had only 250 rounds through it when I purchased it.
This model has a floating pin that holds the mag at it's front. Newer models have a pin that does not float. This might possibly be why I experienced problems locking in some of the "cheaper" mags. Don't know, but then again don't care. I'll be ordering more PrecisionMag units.