I have a cheap Craftsman dovetail jig for my router but have never tried using it. I regularly hear people complain about the lower-end dovetail jigs but praise the expensive, high-end (often $400+) jigs. What are the problems that people have with the cheaper jigs, and what's so great about the more expensive ones?
The one advantage that seems to get people excited? Variable width / spacing.
If you have a fixed width / spacing jig then you either have to design your projects based on the spacing, letting the tool determine the design, or you end up with a partial dovetail.
That partial dovetail may or may not be ugly, especially if you can center it on the piece, but having the ability to space them out based on the project is a real bonus. (It also makes it look more like hand-cut dovetails.)
I have a pair of dovetail templates. They came with a pair of bits to do the cutting. These are some of the 'cheap' ones but still cost me enough.
The first problem I had was they just sent the aluminum template. It was my job to mount them on wooded blocks (for clamping to the board) and adjust them so that they fit snugly (the pins and tails). This took me quite a while and I really only have one set adjusted right, the other is close but still too sloppy. They are hard to adjust because of how they were designed.
The second problem is lining up the board correctly with the end of the template, if you are off you will not line up your ends right. You also need to start on opposite ends of the template if you want both ends of the same board to match the same 'partial' tail or pin. (on top or bottom). So you need to do a bit of extra paying attention to make sure when you are done the boards all go back together and make a square.
I haven't tried using the template on a board longer than the template but I'm sure that will be a much larger headache.