In a nutshell:- That bloke what did the silly cartoons in Monty Python certainly turned into quite the filmmaker.
The Basics:- He started out working as a satirical cartoonist before moving on to being an animator and was "The Python Who Was Never Onscreen Much" in Monty Python's Flying Circus. He took a keen interest in directing along with Terry Jones when Monty Python And The Holy Grail came along, before branching out on his own with Jabberwocky in 1977 (albeit with fellow Python Michael Palin in front of the camera).
Why Are They So Good Then?:- I think that coming from a cartooning/animating background has definitely helped to shape his distinctive visual style. Gilliam quite often combines a grubby, battered, broken-down, realistic look (the crumbling and dust-strewn castle in Jabberwocky, the nightmarish tubes and piping of Central Services in Brazil, Napoleon's war-torn France in Time Bandits) along with the purely fantastical (the Jabberwocky itself, Sam Lowry's winged dream sequences, the Time of Legends) to give his films a real shifting dreamlike quality. He also still knows how to deliver funny stuff - Brazil, bleak as it is, has some great moments of humour as do Time Bandits, Jabberwocky, the much-underrated Baron Munchausen and Fear And Loathing In Las Vegas.
Surely Some Of Them Must Be Rubbish:- Sadly, his recent efforts do seem to have gone off the boil. The Brother Grimm was passable enough but ultimately forgettable and Tideland suffered from having an unlikeable child performer as a lead coupled with a distinctly unengaging story. Some of the blame for this could possibly laid at the feet of his legendary refusal to brook any studio interference (Brazil and Baron Munchausen were both the subject of extremely well-documented battles with the studio while production on The Brothers Grimm was shut down for some time due to arguments with the distributors) along with the almost supernaturally unfortunate bad luck that seems to dog him as anyone who has seen the hilarious but heartbreaking documentary, Lost In La Mancha, will know (it documents Gilliam's attempts to film The Man Who Killed Don Quixote which is ultimately abandoned due to a serious of mishaps).
So We Should Seek Out These Things Of Which You Speak?:- Personally, all of his films up to Fear And Loathing In Las Vegas are absolutely worth a watch. The Brothers Grimm I can take or leave and Tidleand is, in my opinion, really best avoided altogether. If you any interest in surreal, black comedy with a distinctive visual slant and, in most cases, a hefty slice of dystopia then Terry Gilliam films are the ones for you.