The fares are higher than equivalent journeys by rail, particularly through fares.
I'm sure journey times will increase due to extra stops, this is unavoidable when street running commences.
Of course lower safety standards apply to light rail than heavy rail, for example they may be able to drive on sight in certain places thus removing the need for signalling.
As for environmentally friendly, well if they really cared about the environment they'd do it in stages ensuring that passengers could get from Oldham to Manchester by one of the routes at all times, thus avoiding forcing people onto the roads.
I'm pro-tram instead of buses, but when it comes to replacing heavy rail then I think the benefits are exaggerated.