If something is a lot cheaper than the competition there is usually a reason for it. Air India consistently get poor passenger reviews on websites such as Skytrax. Expect poor service, a dirty aircraft interior, shabby cabin furnishings, non working IFE, delays etc.
On the Australia route they use the B787 and have just joined the Star Alliance so things may be looking up a bit for them.
Their business class tickets are often considerably cheaper than other airlines on the same route, I really wouldn't want to fly on them in economy. Singapore Airlines have good India connections through Changi which may be a better option unless you particularly want Delhi non stop (be careful of disruptions due to fog in winter).
Joining their frequent flyer program, Flying Returns will get you an extra 10kgs of baggage allowance just for being a member.