The Texas Instruments TI-30 SLR+ calculator is an OUTSTANDING calculator for me. If you find one, I think you will agree.
This calculator has several features that separate it from being just another calculator. While it doesn't have any "unique" features, each feature it has is well designed and implemented.
First, the size of the unit is comfortable in my hand; so often calculators are too small or too thin to be comfortable to hold. The buttons give the user a great tactile response. The buttons are hard plastic, have a texture to on the top, and are large enough to actuate without hitting a neighboring key.
The keys are also color coded to mark the different zones: numbers, basic math, and scientific functions.
The unit is solar powered, therefore does not require batteries.
Last & definitely not least, the calculator comes with a hard plastic slide on/off cover that is stored on the bottom of the calculator. This means you don't have to deal with the flip & flop.