This extended battery gives you an extra 250mAh of charge, and while that might not seem like much, it made a noticeable difference in my real-world use. If you're a heavy user (like I normally am), you're still going to have to go searching for the charging cable at some point in the day. If you're a light-to-moderate user (and you have a solid data connection), you can expect to get through the better part of the day without needing to charge. (Think leaving in the morning and arriving home after work.)
The one caveat to using this extended battery is the slight (very slight) bump it'll give the back of your phone. The entire package is actually the battery and a replacement door, and while you can certainly discern a difference in thickness the first time you swap the bigger battery in, you'll quickly become accustomed to it. The bigger battery and door slightly round out the back a bit, so instead of there being a slight flare at the bottom of the phone, it's all one thickness.
Fortunately, the thickness isn't so thick that it'll ruin using your favorite cases (probably). All of the TPU cases I tried with the extended battery went on without issue and fit comfortably snug, wrapping up my Galaxy Nexus in a nice, plastic hug.
To give you a better idea of the difference in thickness, we've included a couple of pictures after the break.