I loved the ThinkPads I owned. In the mid to late 90s I was flying a lot around the US and used to carry one of the ThinkPads where you could remove the CD drive after lifting up the keyboard and install a second battery.
That machine also had a removable hard drive that I used to pop out when leaving my computer in hotel rooms so that I didn't mind if the machine was stolen.
On long flights I would carry up to six batteries with me which led to a funny exchange with Nicholas Negroponte who imagined (writing a Wired) a day when batteries had a power meter on them (imagine that!). I wrote to him to say that my solution for long flights was that I numbered my batteries and used them in order. He replied to the effect: "The simple solutions are often the best".
The Wired piece from 1996: http://www.wired.com/wired/archive/4.12/negroponte.html
Now when I travel, almost everyone is pecking away at a
keyboard. The one-line monochrome message has evolved into
a full-color, 12-inch display. That is enormous progress,
but at a powerful price. I now carry eight to ten battery
packs during long trips. I won't even consider a laptop
design that includes unstackable batteries. The fact that
most batteries don't indicate their charge state is
pathetic. It's as if the designer assumed that the laptop
would always be used plugged in, and that people would
travel with one spare battery at most.