They aren't entirely useless... it's just that you really can't do anything for a viral infection except help your body battle the side effects.
3 out of 5 times, people buy the wrong cold medicines. I thought the same thing just last week, did my research (I was a little bored) found exactly what active ingredients I needed for my symptoms and although I wasn't really better any faster, most of my symptoms were barely noticable.
A lot of cold medicines contain "Acetaminphen"(sp) and a lot of the research I've done seems to indicate that although it is the primary active ingredient in most medicines, it's no better than aspirin... it's even debated whether it holds any real value in battling the "common cold."
A lot of cough syrups act as both an Expectorant and Supressant... which means it's supposed to help your lungs remove mucus when you cough... but make you cough less. Am I missing something here? They say it's supposed to make you cough less but move more out when you do, but I don't get why you wouldn't just want to get it out ASAP.
I dunno. You may feel differently, but it's wise to know what you are putting in your body before you just buy whatever box makes the best claims.
Your best bet is to eat healthier and get some rest. As always, lots of water. Orange Juice always seems to help, too... if anything it tastes great.

Aside from that, don't do your cold medicine research while standing in front of all the boxes and bottles in the store.