Personally not a fan, thats not to say I dont think it works, I'm sure if implimented correctly then it could be just fine, but in the main there seems to be 3 schools of sites using it:
1)those who think its okay to stick a pop up box and to start asking me annoying questions the second I come onto their site instead of just leaving me alone to surf - those sites get closed the minute it happens and I dont buy from them - Site Conversion 0 - Customer 1
2) Those who claim to have live help but who have no staff to maintain it so its almost permanently offline - this just irritates me, if you cant maintain it then dont have it, its of absolutely of no use to me to know you're offline - or for you to be online during the hours of 2 and 4pm when I'm not shopping for anything. If you can't do peak times and include evening hours when most people who have jobs actually surf then dont bother having live chat! - Site Coversion 0 - Customer 1
3) Sites who have live chat and outsource the running of it to a call centre abroad! Dont do it! I do expect that if I contact a UK company that I get to speak to someone that can read and understand english to a relatively good level, that they can help me, not fob me off, and not talk to me about irrelevant stuff like the goings on in Corrie or Eastenders in a fake attempt to convince me they are in the UK. Any customer facing element of your business Live Chat included should be able to operate in a way which reflects your company in a professional, helpful & knowledgeable way, and having someone struggling to comprehend the meaning of the question because its not being asked in their native first language is not helpful!
Site Conversion 0 - Customer 1
If you can be there, man it sufficiently with knowledgeable, personable people, 24-7 and not harass customers but leave them to choose if/when to click to chat then yes by all means implement it. If you can't then dont...