I'm not a fan of Mercruiser. I have a pair of 6.2 MPIs and everything Mercruiser added to the base SBC is a pain to work on and mostly stupidly engineered. The best inboard gas engines going now are Ilmor. For older stuff, give me Crusaders any day.
My 2005 Mercruiser has been nothing but great. I do all my own maintenance and find it easy. The hardest thing is the raw water pump impeller. About a 4 hour job start to finish taking my sweet time.