I have two student computers, and my kiddos do xtramath during their morning work time. They don't always all get to go, but most of them do. We started in November, and most of my kiddos made it through addition and subtraction before we all moved on to multiplication.
I use this website: http://softschools.com/math/worksheets/ to generate practice sheets when needed, especially when we do our multiplication sundae challenge. I love that you can customize it--so if you're working on +9, you can make a whole sheet of just those. I've also used it to make modified sheets for my kiddos who need fewer facts.
My district has report card benchmarks that require students to know addition and subtraction facts "with speed an accuracy" to 18. We have a few common assessments and students are given 6 seconds per problem. Most of my students do not score well on the subtraction test.
I do not work on these skills very much in the classroom...Of course, we are always adding and subtracting, but I rarely practice timing them. There are a few games that we play once and a while, and I encourage parents to practice at home. I also have some links posted on my classroom website. (If you would like the link to my classroom website, just send me a PM. )
I LOVE Xtra Math, too! I have 2 students at a time use it during Daily 5 time. It is SO easy to use, and it's really motivating for my students. They like seeing their progress.
Isn't it amazing?! For some reason, my students are really motivated by the paper certificate I print off of the computer when they master their facts (only 2 have so far).
Mine love them, too! I have 3 kiddos who have mastered all four operations and have moved on to doing them with a 2-second timer. The majority are at 80% or higher mastery for x facts, with only a few struggling students around 50%. I can't wait to start using it at the beginning of the year next year!