I think the friend bonus abilities are keyed to both once-per-in-game-day, and also to their current level of "like" percentage. If it drops beneath a given value, they won't help, and they also won't help more than once every 30 minutes or so, which actually seems really long when you're doing several small missions.
As for whether or not it will be cool to like GTA IV again next week, there are a lot of haters here, but I think the game's a big improvement over San Andreas.
In GTA:SA the mandate seemed to be blindly "make everything more expansive and also more detailed" which was accomplished by adding a lot of similar mission types, crazy numbers of go-to/delivery/go-to-and-kill missions, races with tweaked car values, making a big ass map with lots of empty, useless spaces, cars that navigated on rails until you interacted with (bumped) them, at which point traffic turned into the end of The Blues Brothers. San Andreas brought nothing new and successful to the gameplay except turf wars, and even that was abandoned after leaving the first area. On the other hand we were stuck managing CJ's weight and muscle to get girlfriends which (in the case with the SF nurse) fixed a portion of gameplay which had been broken, and we couldn't drive cars or motorcycles or bikes at optimal ability until we'd spent a lot of time doing it -- in other words those game mechanics punished new players and made driving overly easy for veterans.
The driving in IV was really, really hard at first, but I've become accustomed to it, unless there's under-the-hood skill adjustment going on like in SA, only not shown in stats. Now I like driving some of the cars more than the ones in III/VC/SA; the old cars feel pretty simplistic now. Yeah, I can't get away with just e-braking now, but I'm happy they tried something really new.
As for the R3 missions, I'm hopeful that there is enough backlash against their removal that R* puts them in as DLC. Seriously, look at Crackdown. They added group races, armed races, Checkpoint Chase, and several new weapons in their update. I'm really looking forward to seeing what missed chances R* makes up for with the DLC, but I'm also prepared to get what I need from Crackdown (and Mercenaries 2 and Saints Row 2).