I have the LS25 and love it. I bought it for the joinery/template aspect but repeatable positioning, micropositioning and general accuracy make it really fantastic. My workmanship quality and setup speed has taken a real jump with it. For example, I want to put a 1/4" dado 1/4" from the edge of a board. With the LS, I position the fence so it is flush with the outer edge of a 1/4" bit (ie, it would make no cut). Then I move the ruler to align with the crosshair. Now, it just a simple matter of moving the fence 1/2". Since it aligns on increments of 1/32", you get a perfect setup in less than 2 minutes. With a vanilla fence, I'd position the fence with a ruler, test run, tweak the position, test run, ... Lots of stooping and peering and sawdust before you get started with the actual cut.
Frankly, I've never needed the full 25, a 17 would have been just as good. In fact, it's probably a little easier to use since the clamp is closer to the fence. Making a box entails going back and forth between the fence and the positioner base many times. There isn't much cost difference but the 25 takes a larger table/more space. If I was to do it again, I'd probably get the 17.
If you get the LS, check out Furboo's template maker. http://www.routerforums.com/jigs-fix...ate-maker.html
(about halfway down). A whole host of joints are now possible.
By the way, you might also want to check out the wixey (or similar) router height gauge. I have one and it also helps speed setup.
Edit: I think you were asking about the stability of the larger LS fence. It's pretty stable though if you wanted to you could deflect it. The reason it doesn't matter is you don't put very much stress on the fence itself. Even with largish work pieces.