I've been hosting with M6 as a reseller for over a year. The worst problem with them is that their service is email/ticket only, and it is a 24-hour turnaround with the best case scenario. Here's some of the worst problems I've had with these guys.

Intermittent Email for MONTHS
They used athe IMail server, which they claim was causing all sorts of problems and explained the long episodes of downtime and inability to send email. I've got another account with CI Host (I've been with them for 2 years for another domain) and they've been using IMail without any problems at all!

Suspended Service!
Now that I've waited 3 months for them to switch from IMail to SmarterMail Pro and everything SHOULD be working, I woke up this morning to find that they've suspended my account for reasons unknown. No warning, no contact, just a firewall error when you try to hit my sites. Of course, I have to enter a ticket and wait 24-hours to see what they have to say about it!

Bottom line, if you're planning on reselling, do yourself a favor and skip the shared plans like M6 and go with a dedicated solution like rackshack.net or someone that will at least pick up a phone when you call!