Rangetops with griddle
I have a GE Profile with an interchangeable griddle/5th long burner. The griddle is convenient, but I found early on that cooking things like burgers or other proteins really makes a mess. Grease splatters everywhere.
Now, I mostly use it for things like pancakes, grilled cheese, and warming tortillas. Also, the nonstick coating is starting to flake a little, so I guess I should replace it soon. Mine is only about 5 years old.
My 6 burner Viking cooktop with griddle has been a workhorse for more than eleven years. I use it daily but, unlike Philly Ray, I do not cook burgers on the griddle for the reason stated. Ham slices - yes, burgers, no thank you. I also use it as a warming tray. I should note that cleaning is quite straightforward and easy. Whatever brand you choose, make certain the under pan is easy to access.
NB: the griddle is dark brown and shows use, it does not belong in a showplace kitchen. I should also note that the built-in griddle cooks much more evenly, with no hot/cold spots, than the cast iron griddle covering two burners.
A few thoughts in reading the replies so far:
1) I, personally, would not want a non-stick, teflon type surface on a built in griddle.
2) A griddle should be able to be cleaned after use to the original finish. If your griddle is dark brown, or any color brown, it is not being cleaned properly. Every restaurant I've ever owned or worked in had a shiny silver griddle surface EVERY morning when the cooks walked into work.
3) If you want to keep the grease down when cooking burgers, buy a few griddle dome covers with handles on top. Put the burger on and cover it. It will also cook faster...although it's harder to get a pink center this way.