As a 2000 Si owner:

+ Responsive handling
+ More power than stock Civics
+ 8000 RPM redline (trust me, you'll use it more than you think)
+ Lots of parts available, OEM and aftermarket
+ Pretty nice transmission
+ Can be had for $10K

- No low end torque (which is fine for racing, but a bear on the street)
- Requires diligence on your part to keep it from being stolen
- Interior is meh
- Finding an unmodified or unbeaten on one is difficult