I have a Grizzly mini-mill -- as stated, their products can run the gambit on quality.
I've had no problems with mine, BUT.
If you buy one, plan on dissassembling the whole thing and cleaning it, then carefully re-assembling it.
When I got it, the table's leadscrews were out of alignment, and it tended to bind up. After taking it apart, cleaning it, and reassembling it using nothing more than common mechanical sense, its quite nice, with no problems.
Next on my list is one of their mini-lathes.
Now, as far as "good for a beginner", that would depend on what your level of mechanical sense is. Also, don't plan on starting off with hard stuff. I spent a while just making little brackets and such for various other hobbies to get used to the machine and machining aluminum. I'm hoping to start doing some 0% forged lowers shortly after christmas when (again, hopefully) the cash-flow will improve.
If you have any other questions, feel free to IM me.
-FOTBR
(And yes, after I've done some AR work, I'll post a more full review of the mini-mill)