I think driver board failures on Bukobots are fairly rare (much less than 1% so far I think; the Deezmaker guys have a better idea about this I'm sure).
These boards are not hard to find. They are known as pololu boards and various places now make them -- they are open source hardware. They typically range from US$10 to $14 apiece.
For a long time they were based on the Allegro A4988 driver chip, but now new ones based on the TI DRV8825 are coming out. These can handle higher voltages and higher currents, and can provide finer steps. You can still destroy them if you fail to follow directions.
In some cases the new ones can be used as drop-in replacements for the old ones. See documentation about this (for example http://www.pololu.com/catalog/product/2133
, but note that Roy's design may be a bit different). Not sure how compatibility works with the older Azteeg boards.
Basically, remember the warning about powering up with no motors. And, take care if you use voltages much higher than 12V, or if you use bigger stepper motors, or if you increase the feed rates, as these things can increase power dissipation on the driver. Air flow over the drivers will keep them cooler, and sometimes heatsinks might be needed. But a standard off-the-shelf Bukobot with the recommended settings probably won't have problems with this.