By underframe I assume you mean the brackets and Kapex cart + extensions?
I occasionally do site work, so for me having the Kapex cart and associated parts is fantastic. It's a really easy setup, and makes navigating stairs, sidewalks and whatever else is in between my truck and the site relatively painless.
The cart does sacrifice some stability in favor of mobility, which you will see in things like the cart rocking when you do a slide cut. But the motion of the cart is imparted to the saw, and I have never witnessed it affecting the quality of the cut. The one major drawback of the standard mobile setup is the narrowness of the extensions. This can make doing wider boards, particularly those that are not perfectly flat, a bit of a challenge. It's not that you can't do them, but there will potentially be a drop-off from the otherwise perfect accuracy of the Kapex the further your board is hanging over the edge of the extensions.
You can address this however by getting an aftermarket Kapex bracket and making your own extensions:
http://www.multiblades.com/UniversalKapexBracket.html
If I were just shop bound, and cramped for space, I would explore making a mobile station if possible with folding extensions. I did this for myself, adapting the design of [member=11196]Peter Parfitt[/member] :
http://festoolownersgroup.com/festool-jigs-tool-enhancements/kapex-cart/
EDIT: You'll have to search the FOG to find it, but I'm pretty sure I've seen people modify the UG brackets so that they can fit the Kapex on saw stands made by other tool companies.
marrt said:
I searched but didn't find much about the mobile stand and extensions for the Kapex. I have a dedicated shop but don't have the space for a dedicated Kapex station. Consequently, I'm considering the underframe kit. Anyone have experience with this set up? It's a lot of money "for what it is." However, it could solve my problem nicely.