I've been a member since it started.
The membership more or less pays for itself with the free gift, assuming that there is a free gift that fits your collecting interest. Since I only collect SW mini-busts from them, I look at it as more like paying $10 or $20 for the year.
But, the $30 coupon pretty much pays for the rest. I've always found a way to use mine. One year I used it on a Mars Attacks Mini-bust just for the hell of it.
Additionally, you now get points on purchases that can be turned into gift cards for their website, so when I bought the outrageously priced Fett, I was able to knock the price down by about $35 just base don points that I didn't know I had (this may have nothing to do with the Guild, not sure).
The benefit I like the most though is the access to convention and PGM exclusives. They do put out product that is only PGM and the only way to get it is to be a member or have a friend who will help you out.
Basically, if the perks are something you can use, the program is basically free, even though it's gone up by $10 every other year or so.
Also, they do one-off things every now and then... a few years ago they were raffling off the chance to buy limited quantities of really old stuff that they had lying around, and I'm pretty sure that was a PGM thing. I was able to get the ESB Vader Mini-bust for $60 shipped when they were selling for more like $180 at the time.