Disclaimer - I am a rabid SRK fan. I have made my fanaticism clear and public many, many times in the past. I am also a a lover of cinema, and when I review movies I keep my personal biases aside, unless I mention otherwise. I am not wearing my fan hat to write this review. Will appreciate if this respected in the comments.
Okay, let's get started.
Is Ra.One SRK's best movie till date? The answer is a definite no, not even close. Is it even worth spending the time (2:45 hours) and money to go and watch it? Absolutely yes.
The best part about Ra.One is that it does not make any claims of propagating deep messages through a layered storyline, changing the way you look at movies or life forever, or even making you chew over the story after you've left the theatre. It's in your face, visually spectacular and unpretentious. Yes, there is the overall theme of good presiding over evil, but that's more of a side effect of a simple superhero versus super villain story. For it to even avoid that one needed to film leaves fluttering in the air and maybe some critics would have still found that to be too pseudo-deep.
Story in a nutshell (which is pretty much the story itself) - video game developer dad, cool son, create video game with super villain to gain son's approval, supervillain gains AI (artificial intelligence), takes human form and goes rogue, superhero follows suit, in the end superhero vanquishes supervillain and everything is fine with the world.
As SRK and team have been claiming all through their hyper-charged promotions this is an Indian fantasy movie which will appeal to the kids with its world class special effects and simple storyline. And for the many of us who are not kids if the mind-blowing VFX is not enough to hold us to our seats the constant reminders of this being a film that is not taking itself seriously and often poking fun at itself and other popular references (like the initial dream sequence with Priyanka Chopra, Sanjay Dutt and SRK) should be decent value for money. The pace of the movie restrains you from giving in to the urge of checking mails on your phone and most of the gags, yes sometimes racial and stereotypical, will cause some of the people to chuckle some of the times.
The screenplay is fraught with anomalies (such as Karwa Chauth happening before Dusshera) and a curd and spaghetti (!) eating south Indian father suddenly turning out in a Michael Jackson outfit (and driving his son to school in the same) and many more. But the point is we aren't going to watch a superhero movie to dissect the deviations from the realms of reality, are we? In terms of performances I wasn't very impressed with the young Armaan Verma (playing SRK's son Prateek) but that's perhaps because I found his hair to be too long and he portrayed the typical 'smarter than alec' kid which irritates me. Most of the supporting characters such as Sahana Goswami, Dalip Tahil, Satish Shah etc. were caricatures of different stereotypes and were mostly bearable and sometimes funny. Arjun Rampal in the Title role of Ra.One was his usual plastic but good looking self and given that he needed to play a robot he passed off fine, even if not making a lasting, sinister impact as one could've hoped for. Kareena Kapoor looked stunning throughout and climbed many a rungs in my favourite actresses ladder with a seamless performance of a neo-modern wife and mother with the light, naughty side. And did I mention she looked stunning with the few extra kilos? Yes, she did. Finally, SRK. The Subramaniam character came across as a bit forced with no real identity as it was a caricature of the 'average 'South Indian'' (yes, I know there are no South Indians) but he can sleepwalk through these roles, and since he's invested more money in it than ever before the performance remains quite endearing, a bit like Suri from 'Rab Ne Bana Di Jodi' (which remains one of my least favourite SRK film). And as Ra.One he was a mix between himself in My Name Is Khan incorporating a lot of Rizwan and many of the superhero movies such as Terminator, Ironman and even Robot. The fact that he always seemed to underline that he's not taking himself seriously and doesn't expect you to either helped in making it a fun watch which could have easily become a preachy bore.
Bottomline - far from SRK's greatest movie but a fun watch with outstanding special effects, action and moments of humour. A definite one time watch in the theatres. Goes with the usual Indian masala movie warning of leaving your brains behind.
A well balanced review of the movie here by Arnab Ray, a chronic critic of Sir.
And if you really want to know the flaws in the screenplay and why it's not believable then here's a comic strip review by the fantastic Sahil Rizwan.