Well, if you're planning on taking a train or catching a ride with a normal car as a part of your commute, then a folder is pretty much required for reliable transportation without hassle.

I have a 2006 Dahon Mariner (7 speed, modern geometry) and a 30 mile mixed commute with 4 miles of cycling each way. It rides on 95PSI slicks and doesn't seem to give up much speed (if any, really) to my non-folding companions' bikes. The only real speed issue with these folders is the upright stance your body takes on the saddle: there's no cheating the wind on one of these.

There are some other minor issues with riding fast too: folding pedals aren't compatible with cleats/straps/strapless shoes, handlebars are normally flat for folding so there are less hand positions, and expect a folder to weigh a few pounds more.

Still, none of these issues detracts from the enjoyment I get from my folder every day. I think the 2mph I might lose contributes to perhaps an extra 90 seconds on my commute each way. At most. Big deal.

-Warr