Generally, Canon's Battery Grips are similar to each other and the BG-E8 is no exception. This review will sound similar to the other Canon Battery Grip Reviews on this site.
The nice thing about Canon DSLRs without a built-in portrait/vertical grip (all non-1-Series bodies at this time) is that they are small and light. The big downside is that it is not comfortable to shoot in portrait/vertical orientation - especially for long periods of time. And the EOS Rebel bodies are especially small - making it harder to control larger lenses. Thus, Canon makes Battery Grips for all of their non-1-Series bodies.
To install the Canon BG-E8 Battery Grip, the LP-E8 Battery is removed from the DSLR and the battery door detaches. The BG-E8 slides up into the battery compartment and locks into place via the tripod socket screw on the BG-E8. The BG-E8 provides its own tripod adapter threads - this functionality is not lost.
The BG-E8 holds two Canon LP-E8 Batteries that deliver double the number of shots or video time from the T2i/550D and T3i/600D. Alternatively, use 6 readily available AA batteries in the included Canon BGM-E8 Battery Magazine. The BG-E8 still functions fine with only one LP-E8 battery.
The BG-E8 is nicely built - it integrates to the T2i/550D and T3i/600D bodies very nicely with very little flex.
The BG-E8 controls available can be seen in the photos above.
In use, the BG-E8 makes portrait/vertical shooting much easier and more comfortable. I can feel the difference in my arms and shoulder after even a moderately long shoot. And I can hold the camera in vertical orientation a little steadier in this position.
Overall, I think the Canon BG-E8 Battery Grip is very useful - a nice-to-have accessory.