Here's my personal story: I was in a position to have had my GC for about 4 years to when I moved back. In my case, my business was based in the US and I was eligible to apply for a US citizenship. So I went for it. There are three primary criteria for testing eligibility:
1. Continuous residence in the US (you cannot be absent for more than 6 months in a year)
2. Physical presence - you need to have been physically present for at least 2.5 years of the 5 years you were a permanent resident at the time of applying.
3. You need to be a resident of the state (where you are filing your application) for at least 3 months before you submit your application
The process itself is really simple and here are some advice that I was offered which I did not follow. Thought I would share this:
1. You need to be physically present in the US when you apply. I applied from India.
2. You need to be physically present for 3 months in the US prior to your residence. This criteria just means you need to maintain your official residence for 3 months. So if you have been maintaining an an address for 3 months but you were away on business or something, it doesn't disqualify you from applying. Personally - I was in India and Europe for work for 4 months before I submitted my application.
3. You need a lawyer to represent you during your interview if you long absences from the US. Since I was working out of India for the last year or so, I had pretty long absences. However I didn't hire an attorney which would have cost me $1000 or so. I felt that the requirements and procedure was simple enough for me to do this on my own.
Here was the outcome:
1. Application mailed out on May 1st. Priority Date for my application was assigned as May 8th 2012.
2. Fingerprinting done on June 27th
3. Interview completed successfully on August 9th. Oath ceremony completed the same day.
4. Applied for expedited passport on August 10th and received it on the same day
5. Indian visa applied on 13th and received on the same day. I had apply for expedited processing - I had to get a letter from my wife's doctor recommending I was with her as soon as possible since she was pregnant and due shortly.
So the whole process was completed in 3 months flat.
One important aspect to be aware of as a permanent resident or a citizen of the US are the tax laws. Here is an extensive article on the economic times stating the implications of being a resident (for tax purposes) of the US - but being in India.