A review of middle school peer-reviewed studies using qualitative and quantitative methods concluded that when technology (including calculators) is used well:
-Positive effects can occur on students' attitudes toward learning, confidence in their abilities to do mathematics, engagement with the subject matter, and mathematical achievement and conceptual understanding.
-The effect depends on the teacher's skill in integrating technology into the curriculum

Guerrero, S., N. Walker, et al. (2004). "Technology in support of middle grade mathematics: what have we learned?(Third International Mathematics and Science Study)(National Council of Teachers of Mathematics)." Journal of Computers in Mathematics and Science Teaching 23(1): 5(16).