Are you a Firefox junkie, who always wants the latest beta builds? If so, Nightly Tester Tools will be a really useful tool.
The biggest problem with updating Firefox is that some add-ons become incompatible, and take time being updated so they work again. With Nightly Tester Tools this problem is overridden. It's not risk free, but basically it makes Firefox believe all the extensions are compatible, so that you can continue using them. In some cases the extension really isn't compatible anymore, and Firefox may start to behave strangely because of it.
In testing we didn't have any problems with Nightly Tester Tools, indeed it makes updating quite painless. It must be pointed out that it may not always work, although so far we have had no problems. For most people, such an add-on won't be necessary, but if you are using the latest development builds, it could be fantastic.
Nightly Tester Tools will even be useful for people who only do major updates, as it's a great way to patch temporary incompatibilities. A neat way to avoid an occasional irritation.