I have an ICICIDirect, Sharekhan and now a Zerodha account. I am not an experienced trader so with what limited experience I have, I feel Zerodha is the best of them all.
1. Ease of Use : ICICIDirect (Best), Zerodha (Good), Sharekhan (Average)
Trading with a terminal was a bit daunting and I once put a Limit Sell order instead of SL because of wrong drop down choice selection and lost 1.8k. ICICI always confirms a trade so I never made this mistake before or rather ICICIDirect didnt let me. That doesnt mean Zerodha/Sharekhan or Terminal is bad. Just that you have to be careful. Zerodha's terminal was faster than Sharekhans, probably owing to their NSE connection.
2. Brokerage : Zerodha (Best), Sharekhan (Average), ICICIDIrect (Bad)
This is obviously Zerodha's USP. So I wont go much into details but would just mention that brokerage would be easily the least of your concerns with Zerodha. ICICIDirect for all their good points does really bad here with their charges.
3. Service : Zerodha (Best), ICICIDirect (Good), Sharekhan (Average)
Their service is the best by far and when you consider you pay almost nothing in terms of AMC to them (you pay 400 as AMC to IL&FS) its just unbelievable. I call them like 5-6 times a day easily and I can always reach them on one of their 2 numbers. No AVRs and 4-5 min waiting to get to the customer support as in the case of ICICIDirect. However, I dont know how long they (Zerodha) can keep that up.
--Chaitanya