How much you like the game is going to be a reflection of you.
If you cant live with "x" feature from Civ 4 not being in there, you are going to come back here in tears.
If you enjoyed civ 4 but didnt depend on the intricacies to make it enjoyable you'll be fine with civ 5.
There is no better way to put it than "dumbed down" unfortunately, many features have been reduced this way. You can do essentially the same thing but its done in a more generic, less specific way (ie research agreements instead of tech trading, units can move through water on their own but vulnerable) But then again many things have been made more complicated too like combat, building maintenance, social direction, etc. So its give and take.
Gaining or losing is just opinion because its something different.