Well. I like the Marking Map. It helped me a whole lot trying to catch Mesprit yesterday (YEAH, YESTERDAY!!!). It is a very good improvement, comparing to how weary it was to find Latios/Latias in R/S/E and the Legendary Bests in G/S/C.
The Dowsing Machine in the Pokétch is handy as well. Now you have to choose only between registering the Bike, a Rod and the VS Seeker.
The Pedometer is helpful for breeding and evolving via hapiness! The Friend Checker also works for the last case.
In summary, I love Pokétch! XD