Quite enjoyable, and as many people have said, on par with episodes III and IV. But most important: Abrams has developed a trick that avoids a difficult problem of rebooting a franchise, or continuing someone else's. If all you do is remake the movie, people accuse you of making a boring recycled risk-averse sequel. If you actually do something original, people come out of the theater saying "I paid to see a Star Wars movie, and that didn't seem like a Star Wars movie."
Star Wars actually doesn't have it quite as bad as other franchises. If a franchise leans on a paradigm shift driven by a big reveal, there's really no way around it. Ridley Scott had difficulty with this in Prometheus because if you have chest-bursters, people will say "Boring!" but if you don't have body-horror critters any kind, people ask how it can be an Alien prequel. I imagine the people making the V reboot had an even worse time. The Visitors are reptiles? Yeah we already know, it's V, do something original" versus "Wait, the aliens aren't reptiles? What is this bullshit?"
Consequently there's an insoluble knife edge of make something new (and too different) or make something familiar (and boring). The trick is to acknowledge the influence inside the narrative, by having characters recognize how they're influenced. This occurs when BB-8 (obviously this movie's R2) meets R2 and has obvious affinity (by the way, nice cat-like head-bump). Abrams uses this trick the most when he builds a character who obviously is copying Darth Vader to a fault, and has collected his mask. Can we imagine Anakin deliberately styling himself after another Sith this way? This youth and even naivete also gives a nice depth to Ren's character. Yes, there is sort of an "I am your father" moment with Solo and Kylo Ren, but it's out of the way relatively early; and there's a nice contrast when he takes off the mask and betrays our expectation of a "creature" underneath. It should also be added that the fallen star cruiser and AT-AT (and the reverent screen-time they get) are in keeping with the practice of the characters themselves consciously referring back to the earlier films.
As an aside, Abrams did something similarly clever in the Star Trek reboot when Vulcan is destroyed. You're watching that movie and you're relaxed because you think, well obviously Vulcan wasn't destroyed...and then it is. You can hear Abrams at the writers' meetings saying "Hey, we're making a science fiction movie. If we're too hemmed in by franchise canon, well science fiction has time travel and we can do what we want."
All that said, the beginning of the new movie seems unnecessarily derivative. Early in the film the plot is established when stormtroopers attack and, as the smoke clears from the carnage, a masked agent of the Dark Side appears with terrifying powers; against all odds the maguffin (a hologram with critical information) is carried by a hapless droid across a desert planet, ultimately found by a mundane wretch who seems to yearn for something beyond the horizon, and ends up in a dirty trading post. Wait, am I talking about episode IV or VII? No way to tell! The main difference is that the attack happened on the planet rather than on a ship in orbit. Somehow it's the fact that it's another desert planet that annoys me the most. I mean come on, would it have killed you for it to be any other climate? Unless Jakku turns out to be the sneakily re-named Tattooine, that's weak. Second on the list of annoyances is another cantina scene, inexplicably in a cathedral in the middle of a forest. On the other hand, in Abrams's defense, better to err on the side of conservatism than make something that breaks with franchise tradition so early.
As a comment on Star Wars generally: it's hard to call these science fiction films. There are swords. There's magic. There are demi-humans that aren't elves or orcs but might as well be. There is not one lick of interest in how hyperdrive works, or the light sabers, or the economics or politics or history of the First Order and Republic. Which is fine, because it's consistent with the franchise, and that's not what Star Wars is about. And speaking of politics, isn't it a little troubling that basically the fate of the whole galactic civilization comes down to one family with special powers? Much like the Superman franchise, which makes Krypton seem like a third world dictatorship where their whole government really just boils down to a struggle between interstellar Hatfields and McCoys (or the -El family and the Zod faction) who bring their provincial little struggle with them wherever they go. (Yes, obviously it's more interesting when everyone is related and then you sell more tickets or comic books, but people invariably complain when one points out how beholden to the narrative is to the true function of the medium that conveys it.)
I didn't notice any lens-flair, although during the Millennium Falcon chase into the engine of the crashed star cruiser, there's a power-zoom that is distinctly un-Lucas-like.
And a final acknowledgement must be made, of Kylo Ren: forensic psychiatrist extraordinaire. I'm going to start wearing black gloves during intake examinations with my patients.