In the last year I've seen different rankings for online mba... in Europe the best one are always IE and Euro*mba.. Warwick, Manchester, Durham, Bradford come a little distance (with Warwick almost always coming first).. Imperial is even hardly considered.
When 50% leave the course it means:
1) the adcom didn't do a great job (if it's so difficult then require an high GMAT,,,like IE does)
2) the course lacks of interest
By the way some months ago I talked with a canadian who choose Imperial.. he said that this school is better known in North America.. I don't know if it's true but it depends what you search.
By the way ask them to show you their online platform or their sample material and compare with (for example)
openmultimedia.ie.edu/fichas/_dr_mark_i.html