DD 2006 M roadster here. Mine hasn't been any less reliable than my other previous BMW's. Maintenance items are more expensive than for non-M models. If you can do some wrenching on your own, you'll save some big bucks and doing the work is no sweat. There's a ton of room under the hood, which is really nice.
My advice for a prospective buyer would be:
-Early 2006 models may have an older style of bolt within the VANOS system that is prone to failure. Others here might have more definitive date ranges they can share.
-Know what spec you're looking for (colors, trim, parts, etc) and have your money ready. Swapping trim pieces or seats is pricey, so have patience and get what you want the first time around.
-Buy only with a complete, documented service history, including 1,200-mile break-in service
-Make your way through the Zpost forums regularly, as there's a ton of info to be had here
definitely saved time and money from the things I've learned
Good luck!