I've but a magic frothy mug from ebay to mix my shakes at work, its battery operated, press the button and whizz away, you can drink from it then job done... thought it might be better than mixing it in a bottle at work!
I had one - found it good with LL soups, but not the shakes. I just take my hand blender to work, but the whizzy mugs are great for long journeys in the car!
I used it today its on my desk, I used it for my vanilla shake, it was fine, it did not work very well when I added hot water, coffee and the shake, but the water may of been boiling hot and it went lumpy.
Maybe you should try what I do Sarah lou when I'm making the shakes hot - I blend them first with a little cold water, pour it into my big mug then top up with hot water and stir. i add a little coffee and a sweetener to it to to recreate a latte feeling lol.
I'll dig it out of my desk drawer at work and give it another go. It created much amusement amongst my colleagues at the 'dud' i'd purchased, when I tried it before!