Factory Beretta mags probably aren't much better quality than Mec-Gar mags for the Beretta -- because Mec-Gar makes the Beretta factory mags.
And apparently makes their own after-market mags to essentially the same high standards.
Mec-Gar also makes factory mags for S&W, Ruger, CZ, H&K, SIG, Kel-Tec, Taurus, Walther, and others. The are now the sole OEM provider for Beretta, SIG, Walther, and a few others.
Recently, CZ started using Mec-Gar as the sole provider for guns imported into the US. (Don't know about CZs sold elsewhere.)