I have watched Monsters vs Aliens twice, though I did not see it in 3D. Only one theater in the area has 3D and it is the most expensive theater in the area (twice the price of the theater I usually go to, quadruple the price of the cheapest theater that releases the movies after the other theaters have had them for a few weeks), they also charge extra for 3D films. I really would like to see a 3D film but never have. It might have made me enjoy the film more since that was a major selling point for the film.
On my first viewing I did not really like the film all that much. It has a rather cliche plot; which is somewhat expected since it is based on 1950's B movies of the science fiction/horror genre. On my second watch through I enjoyed the film more, though I would not say it is anything greater than an average film. Part of the reason I liked it on my second viewing more than the first is because I expected less from the film and caught references to other films that I had missed on the first viewing. It does have a lot of references to other films. E.T., X-Files, Close Encounters of the Third Kind, Independence Day, etc.
The film did have some fairly humorous parts. For example; I liked the scene with the aggressive girl and wimp jock. It also had some funny lines like this one: "We need our top scientific minds on this, get India on the phone." or this one: "Has it gotten hotter. I'd like to know. It would be a very convenient truth."