Coming to a Mets game can be a pricey experience but it's well worth it. I go to two or three games each season - I live in South Jersey near Philadelphia and catch the Mets at Citizens Bank Park a few times each year too.
I loved our seats in 423 not just for the view we had but also because there were only 7 rows of seats and no drunk idiots that Shea Stadium housed so often. Fans sitting in the 400s will use the same entrance that the fans sitting in the 500s will use, after entering you just walk up or down a set of steps. The climb to the 500s are steep; I've sat up there before. 423 is a much better alternative to it's reciprocal, section 523.
If you're a David Wright fan, our seats stared right at third base for a great view of his play during the game.

Our seats were completely uncovered, so if it begins to rain, you will be out of luck. When I sat here, it was for a night game with pleasant weather.
If you are visiting Citi Field and are going to be cheering for the opposing team, beware. I only witnessed one episode of heckling toward a younger guy who was rooting for the Braves. It wasn't too vulgar, but he had done nothing to deserve it so it seemed out of line and rude. But that was all I saw in the way of harassment. I think Citi Field is a pretty safe place to come and root for another team. Just be polite and realize how tough it is to be a Mets fan and Mets fans likely won't bother you. Yankees fans may be the only exception.