I've seen the Festool guide used and it's nice but I thought it was not really worth the expense since you could make one of these really easily
It all depends, Robert. The Festool (and other commercial) rail system has several major advantages over home-made; it has a replaceable neoprene edge strip which absolutely stops chipping and break-out (until you change the blade), it is both portable and extendable (e.g. you can join 3 x 1400mm or 55in rails to make a single 4.2m or 165in rail - useful for trimming flooring, etc but making the system portable) which a home made guide isn't, it is guaranteed straight whereas timber straight edges aren't (they can and do move with changes in the weather), it has an accurate parallel cut accessory available and there are several accurate right angle fences made to attach to it. But the rail alone isn't what gives the Festool (and its' competitors from Bosch, Mafell, Makita, deWalt, Hilti and Metabo) the edge - it's the plunge action, superior sawblades and excellent dust extraction in addition to
the straight edge which really make the system work so well. And BTW you rarely have to clamp Festool et al
rails down - the rubber strips on the bottom and a small amount of downwards pressure is generally enough to stop them moving.
The major downside is the price!
One thing about the Festool version is that the circular saw has a stepped base (might be an option) which provides a zero clearance cutting surface; however, a stepped base could easily be added to any circular saw to achieve the same results.
The Festool (and it's look alikes) actually have a very accurately machined groove cut in the base (not a stepped base) which also contains cam adjusters to take-up and wear in the self-lubricating guides. In addition the pivoting of these plunge/rail saws means that making a long mitre cut on the rail does not ruin the edge of the guide "rail" as it does on home made version. The plunge/rail systems are streets ahead of a home-made approach in terms of speed of use and accuracy/repeatability. But as I said that comes at a price which puts this type of saw out of reach for many people whilst for many professional carpenters it has become a must have.
For a long time I was a sceptic, but about a week after buying my first plunging rail saw system (Hilti) I was utterly convinced and wondered why I'd been so anti in the first place