I'm a Knight Rider fan (kinda like the way Star Trek/Star Wars fans feel about their shows). I watched the entire series back last year. Comparing it to the original series, I'd give it 3.5/5, especially in an arc that develops towards the end of the series. Comparing it to other contemporary shows on TV, I'd give it a 2/5, maybe even a 1/5. There's a reason why NBC didn't pick it up for another season and now it's in syndication.
The show has some promising stuff - KI3T* is a bit more sensible. No more 'oh I didn't pick that up on my scanners' kind of crap that I hated about TOS**. The car does grow on you but still looks like it has the aerodynamic properties of a brick. There's not much in the way of action compared to TOS. KI3T auto-cruises and there are some cool features, like a windscreen that's like a HUD and shows info on it (incl. video). The surface of the car can display stuff too - it has that stuff that M$ is making for its coffee table.
Most fans are divided on the singular most important issue: TRANSFORMATION. That's right - the car 'transforms'. It will 'transform' into a 4x4 (another Ford vehicle; advertising is as blatant as it was on Team Knight Rider where the Ford logo was prominent on each car). It will also 'transform' into convertible. The mechanism by which it does this is not known but thought to be built on the premise of nanotechnology which was demonstrated in the Pilot show. In the pilot, KI3T has a 'shell' made of nanotech-type stuff which can be altered into different shapes, and was used to make 'minor' modifications to the car, including changing colours. In the series, the entire vehicle can change. It must have a repository of 'nano-particles' to do this. Also, it has machine guns popping out of its bonnet and missiles popping out from doors too small to hold them. Some fans accept the way KI3T transforms. For others, like me, it makes the show less believable.
Stunts: There were far too few stunts. Presumably because of costs of using so many cars and damaging them. In the 80s, a there was a train carrying a load of trans-ams to the dealer. The train derailed and as a result the cars could not be sold to the public. This meant Glen Larson had around 11 cars that he could use for stunts etc. There were more turbo boosts in the pilot episode of TOS than in the entire season of KR08.
In terms of plot, the show has promising plots but these are poorly developed or not taken to its fullest potential and as a result the entire thing feels half-arsed. Halfway through the season, there was a 'reboot' involving killing off half of the cast (there were too many to keep focusing on so lack of proper character development) and this arc stretching three episodes was well done but managed to be wrapped up in a single episode (instead of a two parter which most fans would have been better). Too much T&A (more than baywatch) also helped the show lose its credibility, as well as the fact that the main female co-star was in the car more than she should have been (how many times do you remember Bonnie sitting as a passenger in TOS?), again detracting from the 'one man and his car' feel that TOS had.
Think that covers most of it. Watch it with low expectations and you might be pleasantly surprised. Watch it as a KR fan and prepare to be let down. Don't feel you have to watch it though.
DH*** really wanted to bring back KR. The aim was to produce a series where the lead was a female. The whole thing was crap and is thought of as a separate universe from the other KR spin off, TKR. Again, there were very few KR fans who liked it.
*KI3T is used to differentiate between AIs - the newer Knight Industries
Three Thousand, and the original Knight Industries
Two Thousand, normally referred to as KITT.
**TOS = the original series
***David Hasselhoff
