Just got one of the new Vagabond Mini Battery Packs for my Alien Bee set ups. I had an original Vagabond II (its about the size of a small car battery) and I just hated lugging that thing around.
The Mini is about the size and weight of 3- 580 Speedlights, so it will fit in your camera bag no problem. I also really appreciate the second plug. Alien Bees seems to be making these per order, so if you want one, they need to be ordered a good month in advance. While you wont get the same number of shots as you will from a full size Vagabond II (600-1000 shots depending on power), the mini will get you through just about every shoot (250-500 depending on power). The mini also allows for modular swapping of backup batteries with the terminal.
I have also learned they are discontinuing the Vagabond II, so if you want one of the bigger ones, now is the time to order.
If you already do, or are interested in location shooting with an Alien Bee (think those nice beach shots with the sunset), the vagabond mini is absolutely the way to go!
For more info on the mini, check out the Alien Bee Website:
Vagabond Mini Lithium Battery for Alien Bees