I recently purchased the Pet Allergy upgrade kit for my XV-15. I am very happy with the new HEPA-like filter, I can clearly feel that more air is pushed through and it doesn't need emptying as often as with the standard filters, but the brush is another story.
One positive thing about the pet brush is that it's almost completely silent. The annoying flap noise from the beater bar gone which is great since 90% of my apartment is hardwood floors.
The downside is that it seems to trap hair like a magnet. Every single time my Neato returns from duty the pet brush is full of hair wrapped around it. Not only that but there's a lot of hair in the belt-motor-compartment and stuck to the belt itself. I have to clean it with an upright vacuum after every single run. Compared to the beater bar this is like night and day. I usually clean the beater bar once a week and even then its only a few hairs wrapped around the middle. I haven't noticed Neato picks up more dirt/hair in the dirt bin with the pet brush, so I guess the hair wrapped around the pet brush goes in the bin with the beater bar.
I should mention that I did have a bit of a hard time fitting the brush into my XV-15. Apparently the plastic things on the edge of the brush (the guards?) weren't pushed all the way in when I received it and so it wouldn't fit in the XV-15. I had to apply a bit of force to push them inwards but its the same with the beater bar so I didn't give it much thought. Perhaps I have pushed them too far or too little? The brush does seem to fit perfectly with no space in either end.
I am curious of experiences from other pet-allergy upgrade owners. Do you have the same problem with hair trapping on the pet brush? Also I have tried the pet brush with several different filters (standard, Kobold VR-100, custom HEPA filters from ebay
and the one from the pet kit); none of them affects the amount of hair wrapped on the brush. Perhaps I should mention that my girlfriend has quite long hair (~40cm) and it is almost entirely such hairs that seem to wrap around the brush.
Attached are pictures of my Neatos brush after to different runs. The brush was completely cleaned between the runs.