They last the longest if you keep them off tramp concrete, steel, and other non wood/dirt junk that tends to hide where I mow. If not for that , you can get quite a while out of the teeth, even longer if you hit less dirt (more even ground for example). The mower certainly performs better for me with teeth than without for producing a less coarse finished product.
I've got a couple more jobs to do with my Bradco then I'll be listing it for sale. Mowing/Mulching has never been a focus of my company, and the CTL doesn't get many hours put on it (265 since April of 2014). I am going to be building a grapple truck in a few months which is for the focus of my company (Tree Service, specifically removals) so the CTL and attachments are going to be sold.
The reason why I picked (and will pick again if I need one) the Ground Shark is because the carrier for the blades is massive and it captures the bolt on both sides of the knife putting the bolt in double shear vs single shear (good thing for strength!). I bought mine last year and have no major complaints. About the only thing I can think of right now is making the mounting plate solid so less debris comes back on my power attach cylinder... but that's a minor thing overall. It flat out cuts!