It's worth hitting the Practice +10 button every day
I've been at the Duolingo caper here now about four weeks, slogging away at French. I found out fairly early on that that Practice +10 button, even though it's "attached" to Basics 1, actually contains questions about everything you've covered to date, so it's worth hitting that button a couple of times per session for good refreshers on everything you've covered.
Beware, though, because it will also spring completely new stuff on you. Just now I was treated to entirely new words not covered in any of the lessons I've previously done, which makes things a bit curly, but also fun/challenging.
Very true - it's incredibly handy. I haven't had the same issue of it bringing up new content, but my complaint is that it seems that all of the questions 'practiced' come from the same topic - the topic being randomly selected each time. I assume it would be best to have random questions from random topics/categories, and perhaps this is already the case, but I know I've had a few instances where I'm answering all animal or all food questions throughout the course of a practice session.
Overall - it's a great tool to keep you sharp!
Practice makes perfect, but I've noticed that when I use the Practice button, no skill is added to the progress scales of units. I just get the points in general. That is why I usually practise certain units that I choose myself, thus earning points both for the skill progressbar and for the daily progress.