Nope, it's not a fun button per se...but it will keep the truck in a higher RPM when engaged and helps keep the RPM and torque up to give you more power when towing, especially over hills.
Here's some info from the 2007 Owners Manual:
When to use Tow Haul and O/D Off modes -
When driving in hilly areas, towing a trailer, carrying a heavy load,etc... and frequent transmission shifting occurs, press the Tow Haul O/D button once to engage Tow Haul. This will improve performance and reduce potential for transmission overheating or failure. When operating in Tow Haul mode, 5th gear is disabled and 2-3 and 3-4 shift patterns are modified. Shifts into OD (4th gear) are allowed during steady cruise and automatic closed throttle downshifts to 3rd gear are performed when driving conditions warrant for improved braking.
Pressing the Tow Haul O/D button a second time to select O/D OFF will disable 4th and 5th gear completely.
Your dash indicator light will indicate what mode you are in. You can release the modes with a third press of the button for full overdrive operation. An ignition cycle will reset the transmission mode to full overdrive operation.
I run in O/D off around town and only use full overdrive operation out on the highway at speed. I have used the Tow Haul mode as well, when towing, and it works as advertised. Mileage sucks but the engine is running at a higher RPM when Tow Haul mode is engaged so that's understandable.
I guess it could be a fun button around town in O/D OFF as the RPM's will always be higher vs. full OD operation. I do know I can get my truck sideways in O/D off on a wet street. The reaction by folks nearby when that MegaCab slides out is great to see.