I can tell you from a customer standpoint (not a employee) that some of the tools they offer are great. We utilize their Easy StoreMaker software and DID use their site builder until it got to the point where it was costing more than it was worth to be honest. However, overall I would say very little bumps in the road.
I think you will find very few of Hostopia's customers are on this site. They tend to be larger ISPs and regional telcos and domain registrars. I think they have about 325 resellers right now according to their prospectus including some of the largest telcos in Canada and the US and are trying to expand into Europe.
I would think you could ask them for referrals if you can not get more info here or at other sites and they would provide to you just like you would and should do for any other company you are looking at.
They are likely more costly than some others but obviously many of their customers think it is worth it. Of course, as Hostfrog mentioned, some aspects of their offer may be too costly at some point but it depends on your business model.
Good luck. As I said, I would contact them directly. Fair warning - Today may be a busy day for them as it is their first day as a publicly traded company.
www.bluegenesis.com is a subsidiary of Hostopia. They offer the same technology platform as Hostopia but to smaller resellers. Once you grow bigger, you can easily migrate into a Hostopia platform where you can customize your offering in more detail.