Blasphemy to the naysayers. I feel it's a rather good game whose reputation is hampered by expectations based on Starfox 64. But it's a largely different game! It's still very much a shooter with flying missions and submissions, but it has a lot more variety and non-linearity to the gameplay. I really appreciated the mixture of vehicles with foot-based battles. I love being able to walk around on foot, explore and shoot, then get into a vehicle for a new kind of exploration and shooting. The only problem with control is the tank, and the best tank control is in a different setting from the best on-foot control, which is what I use (dual stick mode).
Besides that, my only complaints would be rather insignificant ones: the sound effects of the ground enemies when they spawn or get shot are kind of weak and unsatisfying. They sort of fade in when they spawn, and when you shoot some things there isn't a sound effect other than your gun. They might fly in the air when they're killed and fade away with some cheap explosion effect, but no splat, crunch, or anything gratifying. In a shooter, this aspect is surprisingly muted. Even running the bugs over in your tank merely causes bump sounds rather than crunches and cracking shells or explosions. Really, it gives it somewhat cheap production values in a way, but you get used to it. And everything else, especially the music, is fantastic (kind of reminds me of Body Harvest for N64 in this way). A lot of the graphics are great, although the insects that appear on the grounds are admittedly simplistic with little animation. Also, the charge-up shots don't destroy nearby enemies as much as in Starfox 64, but it ends up evening out because taking out a group even without charging up still gives you combo points (something SF64 didn't do).
The positives heavily outweigh those negatives. You've got fantastic arwing-only missions in the beginning, middle and end, with highly enjoyable mixture missions in between. The fun part of the ground missions come from the mixture of on-foot and vehicular gameplay. The levels usually require you to switch between them, i.e. take out targets inside and outside of an environment and in the air. A few levels have sections with ground-only missions, then they'll throw the Arwing in for good measure, and in two or three cases you'll fly on someone's wing for a part of the level. These parts turn out to be surprisingly challenging and fun. So you're always shooting, but there's constant variety. The whole level is often a battlefield.
You've got better voice acting with better dialogue than Starfox 64. Not just better, but excellent. The characters feel tangible and less shallow (aka not all bad or all good). Yeah, they're furries, but in Starfox Assault I'll admit that I care about the characters. They were surprisingly good in 1997's Starfox 64, but in SFA they're surprisingly great. Even Slippy has some good lines, like "Why don't you just keep you beak shut for once, Falco, seriously!"
Then there's the multiplayer, which I haven't even mentioned yet. It's excellent. Being able to jump out of an arwing, turn around and shoot down a player-controlled rocket, then jump back into the arwing is a victory to behold. You can do everything but jump on someone's wing and shoot with the pulse cannon (as far as I know, at least). The unlockables are plentiful and constantly livening up multiplayer. I'll admit that at first the arwing-only levels in multi-player seemed difficult at best to play, but once I stopped relying on the break button to target people and started relying on my flying (this goes for the mixture levels, too), it cleared up any problems.
To be honest, I don't recommend this game if you're not willing to try a new direction for the Starfox series. The scope of Starfox 64 is definitely more limited than Starfox Assault, so I can appreciate the different direction they took. Really, who expected the series to stay as a flight-based shooter after Starfox 64? It introduced all-range mode, the tank, and the on-foot gameplay. This game expands on those premises greatly, but a different and slightly rough game it is. If you can't accept this, just rent it or play a friend's copy. If you're gonna play it, do so with an open mind.