With the GDL the first deadline is early February. HOWEVER, in reality there are many, many providers of this course and as such you stand an extremely high chance of getting onto the course anything up to a week before it begins in September.
As to whether it is any good or worth it - usually it's not seen as a particularly fun year but is a necessary means to an end.
If you are interested in becoming a commercial or corporate solicitor in a medium or larger law firm, these firms will hire students 2 years in advance. So if you wanted to begin in 2013, you'd apply in 2011. This means they can pay for your GDL. As such, it's probably better to aim to secure a job BEFORE you do the GDL, as then they will be paying for it instead of you.
But in order to get a job, you're going to need to be looking at getting a hefty amount of work experience. If this is the case, you need to start looking at getting some as soon as the Easter holidays, as law firms interview all the way through from January to September.
Alternatively if this is definitely something you end up wanting to do, take a year out, clear your thoughts, and get some sustained legal or commercial work experience during the time, instead of rushing straight into the GDL.
This is probably more information than you need, and only looks at one particular (though often popular) route into a legal career however.