I've always treated the Nano as a disposable Dual-In-Line plugin module that is cheap (I can get cheaper from China that I can buy the bare CPU locally) that I'm familiar with (I've been using Atmel AVRs for over 15 years - right back to when they first started sampling the AT90S1200).
Accident with a 'scope probe or a bit of wire - bang - pull it out and plug another one in, program it and carry on!!
Very simple to make your own carrier boards using strip board and in-line socket strips.
Starting in 2016 I'd go for the one of the STM32 series CPUs. The STM32F103C8T6 boards for example are less that US$2 (free postage).
Comparing against the Atmel 328p as used in the Uno and Nano:
- 72MHz clock (cf 16MHz)
- 32KHz real time clock (cf none)
- 10x Analog inputs sampled at 1us (cf 8 at 260us)
- 3x 16 bit timers each with 4 channels for PWM etc (cf 1x16 bit, 2x8 bit)
- 64k flash (cf 32k)
- 20k RAM (cf 2k)
- 1x CAN bus (cf none)
- 3x UART (cf 1x)
- 2x I2C bus (cf 1x)
- 2x SPI bus (cf 1x)
- 1x USB bus (cf none)
- 7x DMA channels (cf none)
- 32 bit ARM Cortex M3 CPU (cf 8 bit AVR)
- programmer only US$3 (cf US$3!!)
- 6x hardware breakpoints for debugging (cf none)
Same basic development environment using gcc as the compiler so a lot of code is portable between systems (only the bit that touches the hardware changes!)
The Arduino IDE (which I really don't like) provides a straight forward interface to lots of existing library code to deal with a lot of extern stuff (lcd, 1-wire, RF module etc).
Tools available from ST Micro allow better control of the hardware and are not THAT difficult to handle (STM32CUBEMX is a wizard that writes all the startup code for you and interfaces to the HAL library for example).
Certainly what will use for new projects but I'll continue with the AVR on existing ones a in the main it does the job intended.