I've been debating getting the Blu-ray set myself but decided against the current version. Here's what I have gathered:
The Original Cast:
I only have the 2 disc sets from 2002/2003. The special features here are really well made and extensive. They are not the usual compilation of interviews that too often pass for special features in the industry, and I'm big on special features.
Those DVD sets have the Director's Cut of TMP, and the slightly extended director's cuts of both TWOK and TUC. The Blu-rays only have the theatrical cuts. The other three films are unaltered.
The DVD of TUC is in 1:85:1 aspect ratio while all the other movies are 2:35:1 (rough numbers), but the Blu-ray presents TUC in 2:35:1 as well, although it may be that the director intended the movie to be 1:85:1.
There has been some fuss about the Blu-rays being noticeably tinted towards the blue end of the spectrum, but there is some debate as to whether this is an alteration or a return to original hue.
The blu-rays add additional commentaries, but (as far as I can tell) import over all the features from those DVDs - I'm not sure if the Okudas' text commentaries, which are quite good, made it over. With the exception of the additional commentaries the new special features on the blu-rays all seem quite superfluous and sometimes just silly from what I've read, but again I don't have these.
There is a seventh disc which features a discussion with Shatner, Nimoy, Stewart, and Frakes, hosted by Whoopi Goldberg that is quite good but only available with the box set. I managed to find the whole thing online somewhere, though I have no idea where anymore. It's enjoyable watching them interact, but I don't know if I'd watch it again.
The biggest reason I have NOT upgraded is that TWOK was the only one in the set to have gotten a proper Blu-ray treatment, meaning it was scanned at high resolution from an original film. The rest are what I call "fake-ray" meaning they ran the file from the DVD through a sharpening software. Fans say they are sharper but there is obvious software involvement and not the usual Blu-ray quality. So I'm going to wait for a proper Blu-ray quality scan from film and director's cuts for the super-mega-signature-ultra editions, which we all know are inevitably coming. The 2-disc DVDs are fine until then.
The Next Generation:
I only have the blu-rays. I never bought the DVDs for these out of silent protest: clearly Paramount didn't put much time and money into these movies (IMHO) so why should I? But I caved and bought the blu-rays for TNG because the entire set was $22 on amazon for a couple of days. The special features are mostly the kind that take a long time to tell you nothing, but the commentaries that I've heard so far are good and honest. As I recall the Okudas' text commentaries did not carry over.
Obviously someone with the Blu-rays for all will be better qualified to answer, but that's how I decided.