Vantaggi
After a 3 year "career conditional" period, you are considered a "career" employee and virtually impossible to get rid of. The job security is great, as there will always be taxes. You get a ridiculous amount of vacation and sick time, and off and paid for every federal holiday (10 per year). The salaries are pretty competitive with tax work outside the service, and its pretty easy to move up a grade a year if you apply yourself.
Svantaggi
Most of the jobs, the work itself is pretty boring and uneventful. You are working in an office setting, and the budget is pretty strict, so the offices are usually not up to date and supplies in short supply. If you can struggle through this kind of day, or actually like this kind of work (I am a tax examiner), its a really solid job with also good retirement benefits and a union, even in non-union states.