I'm a subscriber of Vodafone, Airtel and Tata docomo, among all these 3 service providers, i have experienced huge downtimes from Vodafone.
Here are the problems i have faced from vodafone:

1) GPRS doesn't work in my locality (Rs 95 for one month worked for just first few days after recharge)
2) Customer care disconnects call in middle when reporting the problem. (very poor response)
3) Recharging mobile through their website is a big joke (either have wait for 24hrs or doesn't happen at all)
4) They wont give you UPC code easily (I waited for 8 days to get it after repeatedly calling them everyday)
5) I used to have free sms everyday without any deductions before but now i'm being charged Re1 for first sms.
6) If you don't have any GPRS pack and if u accidentally use internet on mobile, you will never be notified how much amount has been deducted for your internet usage.
7) Sometimes network signal goes completely down and have to wait for hours to get it back.
8) And more...