Matthew,
First of, I don't have a pin nailer, but I've been thinking of getting one to add to my collection of nailers. Here's what I found out...
I think there are just three Grex - the older 630, the newer 635, and the newest 645. I ruled out the 630 because of pin length. That leaves the 635 or 645. Although the 645 is larger and heavier (but not by much), I believe they have the same functionally except for pin length. The good news about 645 is that you get an extra 3/8" pin length. The bad news is that users report that longer pins have more of a propensity to blow out the side. That said, the 645 has the extra capacity if necessary, but you don't have to use it.
There's another nailer that I'm looking at too - the Cadex 23g pinners. The bigger Cadex can handle a 2" pin. But more importantly, both Cadexes can fire headless AND
slightly headed 23g pins. To me this is a pretty good advantage over pinners which fire only headless pins. The downside is that the bigger Cadex is about $120 more than it's smaller brethren.
Two oddities that I've come up with about the Cadex. First, I believe that Cadex and Grex are both made in the same factory and, if you look carefully, they are VERY similar. Second, the Cadex only has a limited distribution network. While I can run down to a local tool dealer about 7 minutes away and handle a Grex, it's virtually impossible to get the feel of a Cadex. On the flip side, since the Cadex and Grex are look almost the same, they reviewers say they handle similarly.
Here's a Cadex link:
http://www.floydtool.com/pin_nailer.htm#23hp.
Regarding reviews, the April 2007 Issue of Woodworkers Journal (just out) has a review of 23g pinners. It picked the Grex 635 as best oveall, with the Cadex pretty much tied Cadex CPB23.50, except that the Grex cost a lot less. The review as pretty well written except that they match the medium size Grex (635) against the largest Cadex. If they had rated the Grex 645, the prices would have been more similar, and the result would have been a tie or the Cadex slightly leading.
Also, the July 2006 issue Journal of Light Construction (JLC) also rated 23G pinners. In this review, they rated the Grex 635 against the Cadex CP23.35 which was more apples to apples. This time the Cadex came out on top (I believe because of it's ability to shoot headed pins). The Grex and Max pinners were very close seconds.
Also, I just read another review of 23g pinners (I think by Fine Homebuilding but I can't find the issue.) I this review, they liked the Nikle pinner the best and downrated the Cadex and Grex pinners. Like the Cadex, the Nikle can fire headless and headed pins.
I'll probably buy when when I start finishing my master bathroom. It turns out that I'm installing a lot more wood there. I'm wrestling with whether I want to use a pinner or my PC 18g brad nailer for the smaller trim. If I could use headed pins, then the 23g pinner might work and produce a better finish. If I'm limited to headless pins, then it will probably be the 18g nailer.
In a couple more months, I'll probably have to make a decision.
Regards,
Dan.