There is a market for this kind of thing, and was talking to my son in law about guided circular saw systems. You end up putting out about the same as a good contractor's table saw, but there's a bit more versatility.
Besides Festool, there's a similar system that uses your existing circular saw called EZ Smart Guide System. You don't get all the other features ... like easy plunging and soft start ... that the Festool saw gives you, but it does give you a lot of other benefits. And the system that cuts up to 8' long is less than $200. It was invented by a woodworker and people really like it ... I actually looked into it, but he doesn't have one yet for my left-blade Porter Cable (but does for the standard right blade design). You can see his system at
http://www.eurekazone.com/ but here's a pic of how it can do tapered cuts (if it links through):
