As far as I heard from my friends who were using it, it is not good, I mean not reliable. It worked great first few days, then there were downtime and sometimes it goes down for more than two days. So many switched to other service providers.
Check with someone who use it in Delhi (if they have service there), the above experience was from friends who are in Blore.
At some forums it is given that Reliance fixed wireless broadband is available in Delhi, but their site doesn't seem to have any info. Airtel also has started it's service but it's site isn't having any info either. Only Tata's site show available plans for that service and it's cheaper than Reliance as at 750/month you get 256kbps unlimited connection.