It looks really nice, brightly coloured and full of personality. Admittedly the character design isn't the best: Banjo and Kazooie themselves are endearing enough, and Clanker's great (although he's more level design than character design), but the enemies and NPCs frequently look amateurish and messy. Nipper and his googly eyes are the worst offenders. Still, I'm a sucker for a nice cheerful colour palette, and I think the textures have been sharpened up a bit, which works well. The sound mix is a bit off, oddly. If you want to listen to the music (and you do, because it's excellent), you'll have to turn it up to a level at which the "you got a feather" flute trill will shatter your teeth at 20 paces. Yes this is an exaggeration. It's annoying though.
The game itself remains very enjoyable, despite a number of dubious design choices. To be fair to Rare it's actually less of a Super Mario 64 ripoff than a lot of people make out. The template's the same, but the levels frequently eschew platforming challenges in favour of a more puzzley-explorey approach. I heartily approve of this decision, partly because the platforming controls aren't quite as solid as they perhaps ought to be, but mostly because exploring was always my favourite part of Mario anyway. This aspect of the game is highly rewarding: the well-designed levels always spur you on to see what's around the next corner, and uncovering a hidden passage is as exciting as it should be. Why don't modern games seem to realise this? OK, Oblivion is a game essentially built around this concept, and it's terrible, but there's stacks of unrealised potential in the idea.
The puzzles, unfortunately, aren't up to scratch. They're all trivially simple and most of them mistake legwork for brainwork. Like, I was coming out of Bubblegloop Swamp in crocodile form and wandered down a secret pirahna-water passage to find a boulder that I had to break to get through, which you can only do as Banjo. No problem, I went back, transformed back into Banjo, used the wading boots to get through, and broke the boulder, which revealed another passage - which was too small for Banjo to fit through, so I had to wander all the way back and change into a crocodile again. OK, so I was obviously meant to discover the passage as Banjo, but even so it remains a lot of waddling about to not much reward (I think all I got was a double-egg-capacity cheat, which I can't even remember now).
On top of this, there are too many moves, a lot of which are pretty superfluous (like the trigger+B charge move) or boring (like the wading boots), and the creature transformations usually just give you lock/key abilities like immunity to hazards or rapport with NPC creatures of the same species, instead of transforming the gameplay like they could've done. However, all these flaws, as annoying as they are, are nowhere near as ruinous as they were in the execrable Donkey Kong 64*, and the game has plenty else to offer. In fact, I've been going through it very slowly because I don't want it to end. Always the sign of a great game.
(*I really should get out my old copy of Donkey Kong 64 and jot down a few thoughts, because I could rant for hours about it. It just gets it so so wrong in so many ways. It's incredible. Watch this space.)
PS As regards to the title: Banjo-Kazooie also has an excellent metagame in which you try to figure out what Banjo's yelling when he does his moves. In the same way as Link is clearly saying "Harp! Harp!" when he's swinging his sword, I'm fairly sure Banjo shouts "Appall!" when he rolls. (I also think he says "get off my lawn" when he's doing a crouch jump, but no one else seems to agree.)