If you are using this for educational purposes, be aware the text is hacked about quite a bit. If your budget doesn't extend beyond one DVD and you want as full a text as possible, then the BBC Shakespeare version may suit you better, despite Nicol Williamson's overblown performance.
But this DVD is a wonderful complement to other ostensibly more faithful productions because it really has been conceived as a movie, not a glorified record of a stage presentation (which the BBC version feels like, and which the Judi Dench/Ian McKellen RSC one palpably is). The liberties Welles takes with the text make sense because the visuals are doing the work of much of the language so why duplicate the effort? (And this more usual belt-and-braces approach, incidentally, helps to explain why most full-text Shakespeare films never quite come off.)
That said, it does show its B movie budget roots - the biscuit tin crown isn't overly impressive and the accents are dodgy - but there is a real sense of darkness which feels more faithful to the spirit of the original than most other film or TV versions. Welles as Lear - now there would have been a thing ...
Postscript Jan 2011: Re Welles as Lear, you can now buy a Region 1 DVD of Peter Brook's 1953 cut down version of the play, cutting the Gloucester subplot, for US TV's Omnibus series - a restored version from a kinescope copy, the quality is perfectly acceptable, and it comes with related extras from the series. The company is Koch vision. I cannot speak for the quality of other DVD issues.