I've got the small MacKissic TPH-123 purchased from Northern last February. (They list it as the TPH-122, but it's actually the newer model, meaning that it's cat 1, not cat 0/1.) Had some trouble with the first one delivered--the knife bolts weren't loctited or torqued correctly. One bolt backed out, then got sheared, and damaged the threading. But MacKissic did right by me and replaced the whole unit.
I'm really happy with it. It's about all the
chipper I'd want to run on my little Kubota
BX1870, which has just 13.7 HP at the PTO. The 3.5" limb diameter rating is about accurate, though as noted above, it'll need to be a straight limb to make it to the knife. For me, anything 3" or bigger is firewood. Though I haven't yet used it on leaves, I'm glad it has the shredder hammermill, which is great for anything green, including leafy tree tops.
I bought it to create mulch as much as to get rid of limbs/brush/leaves. My only disappointment is how little mulch you get out of even a large brush pile, but that's the same for any unit. Some would just let me create little mulch piles more quickly.
Jack