IMO the extended versions of the films are a considerable improvement over the originals. The extra material is not simply "extra scenes" - the whole film is completely recut, with many scenes being extended, even sometimes just having a few extra lines added in many different places.
The extended cuts are more or less the way Mr Jackson would have made the films if running time were not an issue, I think; for cinema versions he cut the films to match a pre-determined running time of about 3 hours, rather than making them the length they really need to be.
Not everyone agrees with me about this, and there are certainly one or two ways in which the cinema cuts are better - for example (spoiler, highlight to read) in the cinema cut when the Corsair ships arrive at Minas Tirith, the audience doesn't actually know that Aragorn and the Army Of The Dead are on board them - so, when they appear, it's something of a plot twist. In the extended cut we've already seen the Dead massacring the pirates, so we know it's coming.
But the majority seems to agree with me that the extended versions are, overall, much better. The selection of extras in the 4-disc sets is very impressive too - many hours worth of documentaries, DTS-ES Discrete sound and no less than 4 audio commentaries. The video quality is slighty better too.