Not mentioned in the posts above is the ridiculous amount of rail overhead they take up.
I've since sold mine, but as I recall, each guide requires near 7-8" of rail outside of the plywood, so no go using a 55" rail to cut plywood.
Read the forum posts - there are many many mods to make to these to help make them more accurate, more stable and extend their range. But in stock condition, crazy accurate is not the phrase I'd use to describe them. The cursor arrow is rough and primitive and a fair distance away from the scale it's supposed to read, allowing for some lack of objectivity in the reading. Pretty darn easy to have accuracy only as close as +/- 1/32", whereas with a magnified cursor line, you can easily get as close as +/- 1/64".
Again - there's some great ideas for addressing this as well as other seemingly "beta" shortcomings.
But the main reason I would choose another option or make them myself is the fact that they are quite cumbersome to install and remove from the rail, and when attached, even a 55" rail becomes a relatively huge and tippy affair, not to mention tying up one of your rails for standard rip cutting.
After using them for a large cabinet project and doing most of the suggested mods to improve their shortcomings, I still sold them off as I would prefer a design that simply clips and aligns on the outside edge of the rail instead of having to be installed from the ends. Seneca's version is this style of design I think.
In a perfect (read efficient) world - you'd be able to lay your rail on the work piece, quickly attach parallel guides, make your cuts and then remove them.The Festool design does not allow that.
Julian