I have the Ruger LCP and have been carrying it for the past year or so. I have only fired it at the range a couple of times, but I was able to keep all shots on a 6" paper plate at 10 yards. The LCP is very light, weighing only 12oz with a loaded magazine, one round in the chamber, and including the weight of the Fobus evolution holster I use. It is small enough to be comfortable on my side when sitting in a vehicle, and it won't print even through a T-shirt. I keep two extra loaded magazines in my left pocket, in a pouch I made, and those are also very comfortable.
I keep my LCP loaded with Magtech Guardian Gold .380 ACP +P 85 Grain JHP, rated at:
Muzzle Velocity: 1082 fps
Muzzle Energy: 221 ft. lbs.
The Magtech Guardian Gold 9mm Luger +P 115 Grain JHP is rated at:
Muzzle Velocity: 1246 fps
Muzzle Energy: 397 ft. lbs.
The .380 is 86% as fast and 56% as powerful as the 9mm. That is a pretty big difference in power, but the .380 will still ruin or put an end to a badguy's day - especially with proper shot placement that I am able to achieve from at least 10 yards. Also, I carry my pistol every day, but I don't shoot a badguy every day. If I know I'll be in a questionable area, I'll often carry my Sig P226 in 9mm. Otherwise, I think the sacrifice in power is worth the every-day carrying comfort of the Ruger LCP .380.