I use my 3 year old MacBook Pro. Its 15 inch and its fast enough w 8 GB ram. And yes, i connect to an external monitor, but mostly to multi-task. I use the Mac's screen as its more accurate colors (for me).
I dont use professional calibration tools. Hope this helps.
I've not been using desktop for many years already, currently I'm using a Macbookpro 15 inch retina which is absolutely wonderful, 16GB ram so its pretty fast for me. I've not calibrated my screen though since its just a couple of months old.
I am now using a gaming laptop for photo editing. 17 inch 90% color gamut Full HD screen calibrated with Spyder and with an i7 second gen CPU. 16GB RAM. The softwares are loaded onto the 500GB SSD and all working very well and speedy. Don't think this helps much for photo editing but the laptop also has 2 AMD 7970 GPUs running in crossfire confiq.
The color and effects turn out to be accurate and good enough after publication and when viewed on other desktop systems and Mac screens
using a gaming laptop as well, the razer blade 14" but the 1600x900 display is kinda tuff on the LR5 Develop mode coz more scrolling but otherwise i like to be able to do all my edits on a laptop due to its mobility