Personally, I am really enjoying CitiesXL. It's quite a challenging game to play, balancing all aspects of the economy whilst ensuring the city's population remains happy is good fun!
Most people (more likely angry kids) seem to ignore the fact that you don't have to pay ANYTHING at all after purchasing the game if you don't want to and would rather whine about it. The subscriptions not even expensive! £12 for three months is nothing; 13.3p PER DAY.
For that you get access to online trading, the ability to visit other players cities, you also gain access to the regular new content, of which mass transit system is a part of. People make it sound like you're selling your soul, it's included in the subscription. Online play will also allow the constructions of unique "Mega Structures", which require the aid of other players to acheive. So it's not like your paying for a "glorified chatroom" as another reviewer put it.
Ultimately, I never played Sim City 4, I don't need to do that to know that Cities XL is an addictive, fun game that has a lot of scope for future development.
5 Stars from me.