It's a little big, and a lot expensive.
For the price of a
single Surefire 60 round mag, I can get
6 standard P-Mags, or
8 standard GI mags with Magpul followers.
I'm not ragging on anyone who wants/owns a Surefire mag, but it's not for me.
Besides, if you're going to go a little nuts, why not go
all the way?