It's a really strong performance, although the televised nature of the DVD does detract somewhat from the original stage performance. However, given that this is to be expected, the overall film is good.
McKellan's Lear is unique and visceral, whilst retaining the gentleness and moving qualities that are essential in the protagonist. However, at times, his performance can, I think overtake the actual text itself - it's quite hard to understand what he's saying at times. Nevertheless, another astounding achievement from McKellan.
Edmund, on the other hand, is pretty rubbish. I think his portrayal of a psychopathic brother is legitimate, but his acting of the part is far too over the top. It's essential that a portrayal of Edmund as insane be conducted with subtlety, and this guy certainly doesn't do that.
Luckily, Frances Barber is spectacular as Goneril - she's obviously evil, but with a depth that expands the role very effectively. Romola Garai is a good and moving Cordelia, without being two-dimensional, and Sylvester McCoy's Fool is an excellent portrayal.
Ben Meyjes is a fantastic Edgar, almost stealing the show. His madness is wonderfully portrayed, and the complexities of his character are constantly held throughout his performance, with his aiding of Gloucester's fall an incredibly moving scene.
Overall, a very very good production, and well worth seeing... just.. try not to let Edmund irritate you!