However, THERE are people who "see" purely MP size and that's it! They think that bigger is better and the more MP your camera has, it must take better photos!
Well, to be accurate, the truth is more MP is usually better. As someone on another forum I regularly visit put it: More megapixels NOT BAD! The idea that more megapixels is a bad thing, and can't produce better photos than a similar camera with fewer megapixels, is mostly myth. Yes, skill with a camera and some raw creative talent are critically fundamental to making good photographs, but better technology, even more MP, only enhances what skilled creativity can produce. So long as sensors are not significantly outresolving lenses, more MP is not a bad thing. It can, and usually is, a good thing.
People often get too hung up on the "diffraction limited aperture" of a sensor, however the simple fact is that the DLA is only where diffraction BEGINS to affect IQ for a given sensor AT the NYQUIST RATE, not where diffraction has detrimentally affected IQ. Diffraction always exists, at any aperture, and it always affects IQ. Assuming you go from a 10mp camera to a 20mp camera for the same sensor size, and your DLA shrinks from f/16 to f/8. By shooting a photo at f/16 with the 20mp camera, you are not getting WORSE IQ than you did with the 10mp camera. It might be the same, however often it will be a little better, so long as you are not already far outresolving the lens. The 10mp camera with its larger pixels was a limiting factor in capturing all the resolution of the image projected by the lens, and the 20mp camera is capable of capturing more detail, even if its only slightly more. More MP usually means better, even though there are diminishing returns (assuming all else is equal...reduce some other factor, such as crappier readout electronics and a crappy ADC, and THEN you might get negative returns.)
See the forum post below for a more detailed explanation of diffraction, and why more MP is "not bad", and usually good. There are ISO chart samples for comparison, which demonstrate the effects of stopping down more and more with a 5D @ 12mp and a 1DsIII @ 21.1mp, and the improvement (and LACK of "worse" quality at any setting) with the higher MP of the 1DsIII is quite clear.
For FF digital sensors, and the current generation of Canon lenses, it seems that around 45-46mp is the limit (the same density as Canon's current APS-C 18mp sensors.) Thats more than double the current pixel density of the 5D II, much like the 1DsIII had almost double the density of the original 5D. Gains can still be made, quite possibly a lot of gains. As sensor fabrication gets better, as we move to better microlensing, higher capacity photodiodes at smaller sizes, backlit readout wiring, lower-noise readout electronics, lower noise ADC's, etc., more MP will not necessarily mean more noise. I think the 1DX proves that a high resolution sensor can still be improved CONSIDERABLY. I think the reason the 1DX is only 18mp is more to achieve the insanely high 14fps than for anything else, and I don't see any reason why the same excellent high ISO performance can't be achieved at 21.1 or even 32mp when you don't need high speed readout. There is an ultimate cutoff, where more MP can't produce better images. Assuming we have already pushed lenses as far as they can go, making sensors denser than 45mp at FF size wouldn't produce anything better in the general case. Canon has mRAW and sRAW, which halve the image size and utilize more bayer pixels per RGB pixel to produce a better, cleaner, clearer, sharper photo (kind of like a Foveon sensor.) More MP would be really great for that...an 80mp sensor wouldn't produce better images @ 80mp RAW, however at 40mp mRAW, there would still be plenty of improvement over a 20mp mRAW from a 40mp sensor. Even assuming noise is as "unacceptable" at 80mp as many people claim the 7D's noise is at 18mp today, the 7D produces FANTASTIC images with the mRAW setting...noise almost entirely disappears, where it is often visible at ISO100.
So yes, more MP is generally a good thing, and at worst, NOT BAD! ;-)