The MPL is very different to the ATPL or CPL. It is a course that is almost entirely based around training someone from ab initio directly to the right hand seat of a jet. The focus is on multi crew flying, with alot of the hours done in the sim (out of 240 hours, 70 must be in an aircraft). The hours quoted are minimums, you will only pass when you have shown competency. It is a very new course, and candidates are meant to be trained for a specific airline. There is some doubt at the moment with some providers who seem to be offering "generic" MPLs, which is bending the rules somewhat, and may not be allowed.
I would say at this stage do not do the MPL unless it is for a specific airline that is offering you a specific job.
With an MPL you cannot act as PIC of a single engine airplane. The conversion required to be a Captain of a multi crew airplane is, I understand, as yet unknown.
The MPL is meant to have an oversight board in each country running it. Conveniently, the CAA have yet to set up this board, despite the MPL being available.
The first cadets that went through were with Sterling, but were laid off when they had around 500 hours. As the MPL is airline and type specific, and they didn't have the requirements for ATPL, they were a bit stuck. I understand it is quite straightforward to change airline, if you can find another employer to take you on, but in a recession it is hard enough for experienced pilots to find a job, and employers are still suspicious of the MPL. I think they all did conversion training in the end, to gain a proper CPL/IR to allow them to get another job.
Personally I would say do an MPL if you are being sponsored by an airline (sponsored as in they will give you a job at the end), otherwise do the frozen ATPL as this will mean you can fly for any airline.