So far not much.
In almost 2 years of ownership, I changed front and real brakes and replaced an exhaust mid pipe, which was a then 6 year old original.
Well, other than regular oil changes and an alignment because of a new set of wheels, that's it.
Runs like a champ so far.
It took me about 4 months of searching to find a good unbeaten example with decent mileage. I was looking only for a sedan with manual. I would've settled for any modification such as LS or GS since they were hard to find in good condition, but ended up with a GS-R, which was definitely a nice bonus.
The car is excellent in many ways. It runs great and can handle many souped up imports and domestics even in stock form.
I kept it stock with an exception of a K&N air filter (fits stock box), an Alpine in-dash mp3 player, and a set of slightly wider 16in wheels. That's it, other than a couple of minor interior accessories.
Handling with wider wheels is great, the stock suspension is pretty stiff though, so the ride is pretty rough. But because of the low weight and fairly low center of gravity, it's a hoot to drive through the corners and on curvy roads.
Fuel consumption is around 28-30mpg if you don't constantly stomp the throttle, which is not bad at all for a car making close to 95hp/liter.
On the highway I really miss the 6th gear though, because 5th is fairly short and the engine runs around 4k rpms at about 80mph, which makes it noisy.
After my '93 Accord, it did feel pretty cramped for a while (the '94-01 Integra is based on a Civic platform after all), but I got used to it. I wish there was a bit more legroom, but it's survivable.
Also, it takes a while to get used to the driving style with this engine. It doesn't have too much power at the bottom, so one has to adjust to that. But once you do, all is good.
I highly recommend one if you can find it and are willing to live with the minor flaws it has. The sedan is a bit more balanced because of the longer wheelbase. Altogether it seems fairly bulletproof.