Take a look through these archives. There has been a lot said on the subject in general.
Another place to post this question with people that *actually own* a Catalina 400 would be on www.catalinaowners.com. At least there you will get comments from people that Sail on, live on, live with, etc etc the Cat 400. The problem with this sailnet board is that too often the same people makes the same comments over and over and before long it becomes ''gospel''. Thats the problem with the internet in general. Personally the "admiral" loves this boat, at least the room on it, the aft cabin is positively envious etc, etc. I have heard it is a little faster than the C42 in lighter air, but the C42 can take the bigger air a little better. Not sure if I am in love with the dual helm, but in personal discussions with the few people that have it, they said they have come to really like it and its easy to helm from either tack and it does really open up the cockpit and there is a *little* redundancy built into the system.
If you look at a used boat they have a different and smaller engine than the newer ones. It is fine, but if you want to be able to make hull speed under motor (for whatever reason) then you might want the newer/bigger engine or possibly upgrade the prop to an Autoprop which will tend to get the most HP out of an engine as possible at all speeds.
As for Catalina''s I think they are a damn fine boat. Bene''s are nice too with better woodwork down below, but I like some of the design features for coastal cruising (useability, comfort under sail, etc) of the Catalina a bit better. Plus I feel the Catalina loyalty and Owners groups are some of the strongest you will find around. And it seems the resale value of the Catalina hold up very well.....(But the new Bene 473 is an awful lot of sweet boat for the money).
Also as someone said previously go into the Sailnet archives right here under the "Catalina" list and do a search for the C400.
ps. If you are anywhere near the Rhode Island area I know someone selling a C400 and I know personally that he takes exceptionally good care of his boat. Plus if you want to email him, he would give you the Good, Bad and the Ugly......he doesn''t sugar coat things and will be honest.