Four eps in, and I'm still on the fence. The central plots aren't that appealing, and the supporting characters ... vary. But I hear Felicity will be back in later eps, so I'm very willing to give it a go. (iO9 had a nice piece about how Felicity's presence highlighted the show's weaknesses because they don't have any characters to match what her guest spot brought to the show, and I agree. She did the same for Arrow, but there she was actually part of the regular cast.)
I've never been much of a Flash reader outside of team-ups and the Justice League and stuff, until the New 52 where it became an instant favorite. Let's face it: The ability to run very fast is actually, pardon the pun, rather lame unless you spice it up with dimension-hopping and time travelling. Manapul and Buccellato made it even cooler than that by turning it into a state of mind. That notion of thinking really fast and anticipating consequences just rocked. (Also, Manapul did a great job of visualizing it). It's kind of difficult to dial back from that to the basic running skill so the show leaves me a little not-yet-overwhelmed, but maybe he'll discover the rest in the course of the series.
Also catching up on Arrow finally, and I gotta say, if both shows were starting right now, I'd be Team Flash. Not just because it's closer to the comics, but because the changes they made don't aggravate me as much. Oliver Queen coming back as a henchman-killing vendetta type? Not my idea of a hero. What kept me going was the supporting cast (yes, all of them. That's unusual) and the fact that the same people who recommended it also recommended Person of Interest, and that indeed repaid my sticking around by getting much better after a bland start. So I stuck around until the very rewarding season finale and am now at the beginning of S2 where Ollie's trying to be more of a good guy but doesn't sound always quite convinced when he says it.
Back to The Flash. As for the central whowentbackintimenanddunit:
I'm pretty sure I saw more than one streak in the flashback. One yellow, one red. (Could have a single yellow and red streak, but I don't think so.) Remember how young Barry found himself out in the street all of a sudden? My bet still is that was one of the streaks (either his own future self or another Flash) saving him. Possibly very dramatically making a choice between saving Barry and fighting the other streak and saving his mum. Maybe in the process of investigating his mum's death, he'll go back at some point and end up being that part of the flashback.
Anyway, most shows don't find their own beat until about 6-8 episodes in, so it's unfair to judge it yet. Also: I like how silly the show allows itself to get every now and then. (Can't get enough of Barry falling off the treadmill.)