For me, I did notice a speed increase. I haven't seen anything in terms of having to reposition layout objects, etc. That was about the only difference between 13 and 14 that I saw... speed. It wasn't incredibly faster, but it was a little faster.
Good experience, filemaker's note of 25% speed increase is pretty much on par with the testing I ran, you'll notice mostly in things like tabbing and clicking between objects and keyboard input being faster.
If you already have the upgrade, no reason to hold back.
I upgraded from FMS13 to FMS14 specifically to take advantage of the new WebDirect speed.
The server part of the upgrade was flawless and issue free. At least for me and my clients.
(I wish the client upgrade was also issue free, but alas there have been a couple issues)