ChipKIT is a great substitution board to Arduino. It offers better performance as it is based on PIC32MX320F128 microcontroller based on 32-bit architecture. Microcontroller has 128K of Flash and 16K of SRAM on board. Having Arduino Uno shape factor ChipKIT offers more 42 programmable pins.
ChipKIT like Arduino can be programmed with bootloader that communicates to PC through USB-to-USART converter chip FT232RQ. Digilent has developed an STK500v2 based bootloader that works on PIC so it is easy to program using AVRDUDE tool. Besides that they adapted an Arduino envoronemt to work with ChipKIT boards. It's called Mpide. It also support Arduino boards but it aim is to program ChipKIT boards. Progamming experience is pretty same as for Arduino and even most of examples written for Arduino works on ChipKIT. This is true since there is no specific hardware elements touched like program memory or EEPROM. As you know Arduino is rich in hardware support libraries as all shields are designed for arduino. Late comers like ChipKIT even if they are hardware compatible may have some difficulties with library integration due to different architecture.