I prefer the first three volumes to the later ones.

The hardest volume to find is number 4, which reprinted the Mekanix (Kitty goes to college) limited series. I've been trying to find a copy to replace my single issues but cant find one under $50 in the last year.

Theres often a complete set of trades listed on eBay (it's been relisted and unsold several times) that might be worth having a search for if you are after all the volumes.