I have had some issues with my 07 M5 but overall it is a beautiful car. The SMG is a tricky part! My friend who is an advisor at Concord BMW has mentioned that SMG's are very finicky and he hates how abrupt the transition is from gear to gear. Then, he concluded that mine was quite smooth so it is a 'flip your coin and hope for head or tail" kind of a proposition. Just so you know, one common problem associated with E60 M5's is that the SMG pump fails every so often. There are many nightmare-ish stories online when people's SMG pump stopped working and the cost was 5500 dollars!!! It was more rampant with 06 M5's but I think the later models have experienced this as well. If you are out of warranty then you are out of luck!
The maintenance would be quite regular and expensive but then again, most people don't use this car for everyday errands. If you limit your driving, you may not frequent the shops too often. In my opinion, with the most high performance car made by BMW, this type of hassle is warranted.
One thing I would strongly recommend to you would be to test drive a potential M5 with an experienced owner. Bring a friend or colleague who has owned one. There are so many things to look for and feel for when you are driving this car and if you are just a regular 5-series driver, you may not realize it (learned from my own mistakes). I have learned so many things from my Concord friend that I wished I had known before I test drove the car.
Overall, an M5 is an M5. You can talk about a Ferrari or Aston but I have seen that M5 loyal patrons are most knowledgeable and have passion that ranks second to none. If your car is not a nightmare (knock on wood), you will be a very happy and proud owner. Just don't get too pompous and pretentious:-)