I bought my Civic Si to use as a sporty commuter car (110 miles per day) and to keep that mileage off my 2003 BMW M3 Coupe. In fact, I was amazed at how much the Si and The M3 had in common (such as the drive-by-wire throttle).
Here's my list some of likes/dislikes after one year of Si ownership...
LIKES: 
- Price! (Although I had to call 8 dealers in So Cal to find one that didn't have a dealer mark up!)
- The huge cup holders. And in the right spot too. My '98 Mustang GT had the worst location for cup holders ever.
- The sliding cover for the cup holders.
- Audio-In jack below the front car charger port.
- Storage space in front of the shifter. Perfect spot for my iPod and retracting audio cable.
- Sliding arm rest. Nice touch.
- Volume, CH/TRACK, and MODE controls on the steering wheel (a MUST if you are used to driving a BMW).
- Attractive interior. Nice seat materials.
- Nice, easy-to-read digital readouts on dash.
- Speedometer is high enough to practically be a heads-up display. Can read it without taking eyes off road.
- Thermometer readout in dash (also a must-have if you are used to BMW's).
- CD player supports MP3 and WMA files! I wish my BMW had that!
- Moonroof switch operation and feel is identical to the M3)
- The way the bac on the RPM gauge fades in when you start the car. The red gradient graphic makes it appear like a sunsrise.
- Two car charger ports! One for the iPod, one for my cell or GPS! Nice!
- Telescoping steering wheel! I'm 6'2" and mostly leg, so I have to slide the seat pretty far back. Having a telescoping steering wheel makes all the difference in the world.
- Factory Trunk tray is nice.
- 8 air bags!
- Fun to drive. Spirited and nimble.
- Great MPG.
- Not a million of them on the road (yet).
- Can program the driver's door to lock when leaving and unlock when turning off ignition.
- Oil Life gauge, and the fact that the user can reset it if they do their own oil change like I do.
- Headrests for rear passengers. Even in the center rear (which will probably never be used).
- Red embroidered "Si" and red stitching in seats is nice.
- Flip down coat hangers on rear pillars (instead of just an ugly plastic hook like cars of yesteryear).
- The little red LED next to the moonroof switch which lights up the shifter area at night. I'm thinking this was ripped from BMW since my M3 had the exact same thing -- but much brighter.
DISLIKES: 
- Price! (I had to call 8 dealers in So Cal to find one that didn't have a dealer mark up!)
- Paint scratches if you just look at it wrong. A stray cat hopped on and then off my fiancee's '07 EX and left very noticeable scratches on the trunk lid. I've since installed 3M clear bra material over the front 1/2 of the hood, fenders and headlights to protect it from rock chips.
- Xenon headlights not available (once you go Xenon, it's hard to go back to Halogen). I may go with an after market kit down the road.
- The oil filter on the Si in the WORST place for access compared to our 2007 Civic EX. The EX was cake. The Si required me to remove my right-front wheel and get a special filter wrench for removal. Total pain!
- OEM fog lights are a rip off and appear to be a pain to install based on the directions I've read.
- Factory mudguards are better than nothing, but I still get water splashed up on the side of my car when turning.
- The shifter knob will feel like a branding iron on your palm if left out in the sun to bake.
- The windshield is so huge, you MUST buy a custom-fit sun shade ($30 on eBay).
- No place to store trash (like gum wrappers) since there's no ashtray.
- Wimpy horn. It's going to be replaced soon.
- Insurance coverage is MORE than the BMW M3 costing 3x the price! WTF!?
- Side mirrors do not fold in for protection against breakage.
- Front passenger seat does not always "lock" back into place after using the rear-entry lever. Often feels like there's a bolt or two loose on the floor bracket. I have to mess with the seat-adjustment bar while sliding it forwards and back repeatedly to get it to feel locked down.
- Front driver's seat does NOT slide forward like the passenger's does when you pull the tilt-lever. You have to manually slide the seat forward to get to the back seats by using the seat adjustment bar under the front of the seat.
- Pulling down sun visors require an insane amount of force, but they swing left/right too easily. Can't tell you how many times I take a left turn at, say, 45MPH or faster and the sun visor which was up against the driver's side window swings right and hits me in the head.
- Black cloth interior shows white lint and other debris too easily. Thank God I don't have a dandruff problem!
- Interior front door panels scuff really easily with shoes. One must enter/exit the car with care! I installed 3M clear bra material to the door sills to eliminate shoe scuffs on the paint.
- The passenger seat belt clip sometimes rattles between the plastic of the interior and the plastic side-airbag cover when driving on bumpy roads.
That's all for now. Will add more later as they come to mind.