First of all, Terminator 1, 2 and 3 are among must watch movies.
as an action movie it's good, but as a sequel for Terminator... not that good. surely it's not better than Terminator 2. T3 doesn't have that well written screenplay like T1 and T2. seems it's just made because they knew people would watch sequel of Terminator. Jonathan Mostow's movie is not comparable with James Cameron's movies.
Nick Stahl don't feel like John Connor (no similarity, even different personality!). T-1000 was an unforgettable character, but T-X is not that interesting. even the terminator itself don't feel that good.
Watch it, just because it's sequel of Terminator.