Just got done doing some testing on a newer app called Blackboard Math. Allegra, our first grade teacher, checked it out as well. We liked the blackboard style - the student is presented with an old-school blackboard with an equation on it; we eventually figured out that the practice page could be adjusted for '1 digit' calculations like 6+2. The addition, subtraction, multiplication and division problems would be valuable practice for students - this would be an effective app up to 4th or 5th grade. Overall, the interface is clean and easy to use.
There's also some cool tracking available- it's set up to support multiple users on the same device, and you can look at a calendar to get a historical view of correct answers, and amount of time spent. Some good stuff there. There are some other cool tracking features - one is that screen shots of actual problems - what the students wrote in the 'blackboard' space - can be viewed later.
Once I got my head around the interface - for instance, understanding that I couldn't actually enter the info as I wrote in the slate area (#1 in image below) - I was good with it. There's a keyboard that appears (or can be locked 'on') up at the top - once the student writes out their work 'longhand' in the slate area, they type their answer up top (#2 below). It was easy to use.
No real frills in the app (a good thing, to me). It's designed to focus on actual practice time and getting better at math - no fireworks, no dancing elephants, and it's not a game. It is, simply, an effective way to practice math facts.
If you are looking for an Android app (with no ads!) that can help students practice math facts, this is a good possibility. There is a free and paid version, too; the free version still has some valuable capabilities. Check it out!