Few months ago I was using Tata Docomo but due to weak network I decided to port in Idea. There is no problem of coverage.
Previously when I was using Tata Docomo there was lot of call drops between Thane to Parel stations. Idea 3G data speed is also good in Thane.
I am using Idea for last 4-6 months but there is no issue with data speed and network coverage. I got 399 plan which is having 550 mins free per month and 1GB 3G data. After 550 minutes they charge 0.30p/min calling charge to idea and other mobiles. This plan is sufficient for average user.