Generally my experience was OK. The prices are expensive as you are paying for on site parking and I've found Manchester to be far more expensive and less practical than Birmingham but not everyone has the choice.
Having stayed on site and off site with a shuttle I just don't think the high prices can be justified. Often, despite what terminal you arrive at you may have to go to 'the Station' area which is may be quite some distance away and to stay closer is even more expensive. Compare this to say Birmingham where I paid less than £60 for three weeks parking just a short 5 minute walk away prices are more than triple. Overall the staff were friendly (you left your key) but there was a large queue which wasn't mangaed well. Staff had the car ready when I arrived back so it was easy to depart - that said it was late in the evening and it was quiet.
If you sign up to Manchester Airport you can frequently get discount codes which can help to reduce pricing further.