I had sent a courier containing important documents through ST and it had reached them in 3 days. I would not say they are fast but they are effective.
For eg: I did try DTDC first but area I wanted to send the consignment is not in their scope. So I went to ST and they accepted it.
Src: Bangalore
Dest: Village near Mannargudi in TN
Tariff: 40 rupees(which was way cheaper than many service providers esp for the document qty of mine)
Qty: 200 gms
I usually go with Speed Post but due to my inability to reach PO within 3 PM, I could not post it through speed post.
Disadvantage with ST: No tracking on Net possible. You need to note the ph no. of booking office only and try to track.
Advantage: If you send any courier to TamilNadu then this is really best because they belong to TN itself so they cover many parts of that state.
Disclaimer: I do not know if they are good outside TN.