I used to have proper systems in my old cars. Now I'm told I'm too old for that...
I had been missing the low frequencies in my music, so I bought the 50w 8" kit awhile back because it was cheap and could be removed easily if we needed the trunk space. I knew going into it that it wouldn't be like my old systems, but I got what I expected.
A plus is that it hooks up to your existing car stereo without any modifications. It comes with these splicer things that tap into the existing speakers wires behind your head unit as well as the power, ground, and ignition wires. You'll still need to attach a ground wire to the car's body somewhere in the trunk. I was able to do this under the back seat. Since it's only 50w, you don't have to run a power wire to the car's battery. The only place you can see the wire in my car is coming up from behind the back seat to the tube.
The tube is sturdy, but you may have some trouble getting it to stay put. I used a combination of velcro and a small bungie cord to keep it from rolling around. If you're ok with drilling holes in the floor of your trunk, you won't need a bungie cord as the mounting bracket is designed for this and the tube itself can still be removed easily even with the mounting base secured. (I was going for the least amount of modification as possible)
The bass is definitely more audible in my car, but it's not Earth rumbling or anything. I mean, the rear-view mirror will vibrate a bit, but that's about it. It doesn't handle a very wide range of frequencies very well and can sound distorted at high volumes. This distortion (papery sound) is less noticeable when it's in the trunk and the back seat is up. Some songs sound great and others not-so-much depending on the frequency.
Basically, only buy this if you're looking for a cheap means of enhancing the lower frequencies in your ride. For $99 you're getting the speaker, enclosure, amp, wiring kit, and mounting bracket. A pretty good deal if you're not expecting the world.
Edit: seems the 8" powered kit is sold-out. tough luck...move along...