Season 5 is straight up weird. It's rich with hints and clues that not all is well with House, and the season finale ends on the mini-mother of all cliff hangers. Season 6 opening episode is going to be unmissable. American tv shows always end a season with a cliff hanger, but so often they're appallingly slapdash. Not so with House. From about mid way through the season, it's subtly hinted at that there is something profoundly amiss, and it culminates in a dark and surprising final few scenes.
Anyway, back to the beginning. At the start of the series, Amber is gone, and so is Wilson. In his place there's a private detective, Lucas (played wonderfully by Michael Weston) who is sardonic and clever (actually a match for House) and, apparently, is being given his own spin off series. That's officially Good News.
Chase, Cameron and Cuddy get more of a look in than they did in season 4 - it's no longer incongruous that their names are in the opening credits... although, Cameron and Chase are largely spent fine-tuning and/or imploding their relationship. Season 5 also introduces a relationship between 2 more of the main cast... and the violent loss of another. Apparently in the States it was widely touted as a "Very Special Episode", with ramifications that would ripple throughout the rest of the season. They set up a bit of a red herring leading up to it, so it is a genuine shock but, somehow, that whole storyline is a little unsatisfying. That being said, House's situation is possibly predicated on it, so maybe there are huge, House-shaped ripples we've yet to fully see.
The team still solve the unsolveable, House still offends anyone with ears (and, indeed, someone without), and it's still fiercely clever and frequently funny. The entire season feels a lot more chaotic than the others - almost as if they wrote much of it on the fly, but it's no poorer for that. It's still House, it's still wonderful, and it deserves no less than 5 stars.