Their cabinets are pretty solid, 1.5" MDF on many all the way around. Their prices are also pretty good.
A great many of their kits are not their design, but are by noted designers like Joe D'Appolito and Orca, or Vance Dickason. So it kind of depends which kit you are asking about. Zalytron pieces together kits per these outside design sources specs, packages them with decent quality components and offers cabinets to match.
I bought my drivers from them, but sourced my XO components elsewhere as I wanted different stuff, and am making my own cabinets. If you have questions about a specific kit, ask at the Madisound discussion baord and you are almost sure to get feedback.
As far as buying from them, though, no complaints.
Northcreek and BESL both completely design their stuff and are both very good at XO design and provide their own selected XO components, so you are talking about kind of different animals vs Zalytron. Which shows in the price difference.
BB