@Oprawindfury -
There are two major reasons I don't recommend CMYK to visitors to this forum. The first is that most visitors to PSG have very limited experience with Photoshop. Many (most?) don't even know how to use the basic tools yet, let alone be familiar something as advanced as color management, color profiles, and certainly not the intricacies of commercial printing. IMHO, recommending "CMYK" is not a good idea.
For example, which variant of CMYK should they use (see attached screenshot)? What about dot gain, total ink limit, type of separation, black generation curve, and the other parameters that are needed to get reasonable looking output from a particular press. Of course, newbies won't have a clue about any of this, so, of course, you have to tell them to contact the printer and ask for the ICC profile that that particular press uses. Then, if it's a big operation with multiple presses, each with its own slightly different ICC profile, there's a good chance the printer will simply say, "send it to us in sRGB or Adobe RGB, and we'll do the optimal conversion".
The second major reason I don't recommend CMYK to visitors to this forum is that most of them are talking about very small numbers of prints (...maybe just one) that they will have done by an on-line printing service (eg, Mpix, etc.), or by the local Office Supply store or some local mom-and-pop parcel store. 99.9% of these operations use inkjets, color lasers, dye subs, or something similar, but never an offset press, because all their business is short runs. The printer driver software for each of these types of printers is highly specific to that particular model, mfgr, etc. It expects RGB input and does an optimized conversion to the inkset recommended for that printer. These days, this is often many more than the 4 basic CMYK inks. In the best of circumstances, if you send the printer driver some ICC version of a CMYK file, the driver software will be smart enough to first do a reasonable conversion to (say) Adobe RGB, and then do the conversion to the (say) C1C2M1M2Y1Y2K1K2K3 flows that this particular model of inkjet needs to produce good output. So, again, by suggesting CMYK, at best, you've wasted their time, and most likely, you've actually reduced the quality of the output they will receive.
Exactly as Larry, ALB68, pointed out, these days, the best recommendation (at least to most visitors to this forum) is for them to work in sRGB, a least-common-denominator color space and let the printer do the conversion for them.
Tom M
PS - For those users who actually need to use CMYK, I just found a nice, fairly simple article about how to prepare such files. Check it out - it's a nice read:
http://www.damiensymonds.com.au/art_newsprint.html
