I think each campaign was cool in its own way.
Leon and Helena went back to the "roots" that fans wanted with (more vicious) zombies and darker atmosphere while also exposing The Family as possibly future antagonists with members in other high positions of power in the US, if Simmons was any indication. It also leaves things pretty shrouded in mystery like what Ada will do after this, and whether President Benford's wish of revealing the truth of the US Government's involvement with Raccoon City will ever come to fruition and such.
Chris and Piers were more the opposite of Leon and Helena in that, rather than "going back", they went further into the future of B.O.W.s with the J'avo. People seem to forget that things like the J'avo were the types of B.O.W.s that Umbrella were striving for since they started researching bioweapons (the zombies were simply failed subjects that didn't turn into Tyrants, but Umbrella didn't seek to create them with the T-Virus) in that they were powerful, but controllable and easy to produce unlike things like the Tyrant (difficult to mass produce, especially after losing Sheena Island in Survivor, much less finding compatible test subjects) or G (very powerful, but too uncontrollable), and what ones they did succeed with had limitations like the T-102/Mr. X/Ivan, and T-103/Nemesis in that they could only follow pretty straight forward programming (seeking the G-Virus or killing S.T.A.R.S. members) and once they started mutating, their programming was effectively nulled and they go back to being wild beasts. Even T.A.L.O.S. required a freaking super computer to be more lethal, but also required limiters (its armor) for its mutations like the T-102 and T-103 lest it run wild.
In terms of the people, it also attempts to be a passing of the torch and could also be looked at as a way of how Resident Evil itself has evolved since the beginning; from slow, mindless zombies and young, confused protagonists to fast, intelligent J'avo (even the C-Virus zombies are slightly more intelligent, if only a little) and battle-hardened, matured and changed protagonists. I mean, we've grown alongside these main characters for so long that I'm sure it's difficult to imagine "Resident Evil" without them, but we also (should) know that they can't keep up the fight forever either as they're only human. Eventually, unless bio-terrorism is wiped out forever, which is doubtful, then younger, newer people will have to take up the mantle and continue the fight in their stead like Chris wanted with Piers ( =(Â ).
Jake and Sherry was more like a "legacy" campaign. After all, they happen to be the children of two of the stars of Umbrella when it was around and both ended up becoming monsters by the end who had to be put down due to megalomania and/or obsession (Wesker and William Birkin, of course), so this is sort of a way of showing that blood alone doesn't dictate the descendants of following the same paths as their predecessors. Whereas Wesker and Birkin's work would've lead to ending the world, Jake and Sherry ended up playing roles in helping to save it (for now). I do wish we got a bit more on Jake's backstory, like how his mother ended up Wesker's lover in order to have Jake to begin with. I wouldn't be surprised if Jake ends up returning in a future installment given it's said that the BSAA is searching him in hopes of more research on his blood for better vaccines as the Anti-C vaccine isn't a guaranteed inoculation, and only works if the person isn't already infected with the C-Virus.
Ada's campaign is really meant to help piece together things that went on behind the scenes during the three primary campaigns and, much like Ada's campaign in RE4, shows how certain things are how they are when you get to the same locations during the three primary campaigns (like the three key cards on the carrier that Chris and Piers have to use). While not as deep plot-wise, it did help sew the holes shut in the three primary campaigns that they couldn't do themselves.