As a little research project I've mined altcoins for a limited amount of time (three weeks) to test myself what I have read online about altcoin mining. My experience is as follows.
I mined Bitcoin, Litecoin and Vertcoin via the large pools and found it is basically useless unless unless you use ASICs mining rigs (which can cost you several hundreds or even thousands of Dollars) as otherwise you'll never get anywhere near the hash-rate required (currently you'll need at least 100Mhash/s, meaning 100 million hashes per second) to compete. This means mining with your desktop CPUs or GPU is an exercise in futility.
The story is a bit different for coins like Monero or Bitcoin Gold where you can actually earn some coins with your desktop GPU (your graphics card), but unless you have a GPU mining rig consisting of several mid- to high-end NVIDIA or RADEON cards, the process is slow and the costs for your power consumption will most likely eat up what you earn several times.
The most viable way for desktop users to earn some altcoins is by sharing their desktop's or server's processing power with scientific projects like SETI@Home, Einstein@Home or LHC@Home among many others. To participate is simple enough and consists of installing a sofware called BOINC and joining the Gridcoin pool. This way you'll get a compensation in Gridcoin (symbol GRC) for every computational tasks your desktop performs for the projects you registered for.
What's your experience in this field?