I'd extend that to the whole HP calculator lineup through about 1994 or 95.
There are a lot of aspects I could single out, but William Kahan's involvement in their numerical algorithms is a particular standout for me. They wanted high quality numerics out of battery powered 1970's era micro-electronics, so they brought Dr. Kahan in to design the algorithms necessary for the task.
(Dr. Kahan is also noteworthy for his involvement in IEEE-754, which is still the standard for floating point today.)
Serious, serious engineering went into these calculators.
I'd extend that to the whole HP calculator lineup through about 1994 or 95.
There are a lot of aspects I could single out, but William Kahan's involvement in their numerical algorithms is a particular standout for me. They wanted high quality numerics out of battery powered 1970's era micro-electronics, so they brought Dr. Kahan in to design the algorithms necessary for the task.
(Dr. Kahan is also noteworthy for his involvement in IEEE-754, which is still the standard for floating point today.)
Serious, serious engineering went into these calculators.