I find that pings aren't that different. However, if you have a poor quality connection - either due to interferrence, distance, or crap hardware, you run the risk of dropping packets, which have the effect of lag spikes.
I had a really bad USB wireless card for awhile. It was so terrible that whenever CPU usage hit 100%, it would disconnect.
I replaced it with a newer Linksys, and it worked a lot better. Ever since then I keep wireless far away, and insist on hardwired ethernet.