Drive one. Everyone has different opinions, as for tame compared to other M models they couldn't really be any more wrong. The 0-60 time is more like 4.7 as confirmed by BMWCar magazine, the ride is hard and the clutch and steering is heavy but all M cars are. Ive driven the E46 M3 E92 M3 and own the Z4M I have to say the experience is very different but I would say the Z4M is the most fun. These cars are for very specific people thats why reviewers were not very warm toward them. As for servicing its more expensive but wont break the bank, and is quite frugal if you drive it nicely you can see 30mpg, drive it hard and you see lower 20s but mine over the last 1000 miles has recorded 26.6 ive only had the car 3 weeks and love it. Its definetly one of those make your own judgement cars.
Sounds like the reviewer didn't like the car from the off anyway. Have a look at EVO or BMWCar, better reviews.
http://www.evo.co.uk/carreviews/evocarreviews/64720/bmw_z4_m_roadster.html