The C5.1S is a shielded center channel speaker.  This was one of my first speaker projects.  It uses some pretty simple components and a very simple box design.  The woofers are 5" shielded Radio Shack cheapos.  They were designed specifically for "older" center channel applications.  In the early days of Pro-logic, the center channel was intended primarily to reproduce voices - usually from about 100 Hz and up.  Since then, the center channel speaker has become one of the most important speakers in a home theater, containing up to 55% of the sound information in a movie.  This driver has a relatively high fs, so it definitly won't play low.  The Vifa tweeter makes up for the woofer's defeciencies.  This is a great tweeter.  It has great range, great power handling, and a great sound.  It can get a little harsh if your receiver or amp has too much gain going to the highs, but other than that, it produces a really clear and clean high end.  The first iteration of this speaker didn't use a shielded tweeter, and believe me, I paid for it.  After nearly ruining my new TV, I decided that it was in my best interest to get a new tweeter.  Trust me, don't even think about using non-shielded drivers anywhere NEAR your TV.

This is a nice sounding center channel speaker.  It can handle a hefty dose of power.  It does a great job with dialog and offers a good clean sound.  The midrange is quick and detailed, and offers great dispersion.  This design could stand an updated woofer configuration.  There are quite a few good models out there that are just about as cheap (the 5-1/4" Dayton unit from Parts Express, for example) that would offer better bass performance.  Of course, new drivers means a new box design.  Also, a cheaper tweeter could be used, which could cut the cost of this project even further.  Parts Express offers a great shielded titanium dome tweeter for about $10 less, that has a really great sound.  Overall, this is a cheap alternative to buying a junky center channel speaker from a retail store for twice as much.