I just arrived today morning into Istanbul airport from Mumbai and got the visa on arrival done. Here is the run down.
1. It is very easy to get it. You have be a bit patient. It takes about 30 to 45 minutes to get it so dont plan a quick connection.
2. The visa is NOT available from the normal visa counter.
3. You have to go to a special police desk which is at both the of the large and the small immigration area.
4. The police will check you passport and then give a paper which mentions the fees etc on it.
5. You take this paper and your passport to the main visa counter where they issue all other visas. There you pay the money; 15 euros or 20$. And they will give you a receipt.
6. Take the passport and the receipt back to the police desk as mentioned above. The police will them complete the immigration at one of the counters next to the desk and then you can enter.
7. Ensure you tell them exact number of days you want to stay as they are issuing visas based on that. One guy in front of me was not sure about 6 or 8 days, they gave him a 6 day single entry visa.
8. There is no requirement to show them any invitation, hotel reservations. But better to have them handy.
9. The funny part is its cheaper to get visa on arrival than from the consulate. Costs Rs. 2700 to get visa from the local consulate.
Overall there is no problem in doing this when you arrive.