Auto responders are key, but you must deliver on what you promise. If it says you will respond for example "in the next four hours", then you may be making a grave mistake (mostly because when people fill out a contact form, it tends to be after business hours). All I am saying is be smart with your wording.
I totally agree with Sergio...only promise what you plan to deliver!
I personally have auto-responders on my website forms and I use auto-reply on emails when I'm away from my desk. I think both are great tools so you don't keep the customer wondering when they're going to hear from you.
I have never used an auto responder. I have my mail sent to my blackberry when I'm away so I can respond immediately to any inquiry.
I don't have an A class property. We don't have abundant parking or washers & dryers, or a big budget. So I sell what I DO have. Customer Service with a smile, anytime. When I leave my office, my phone and email are forwarded to my cell. My residents know they can reach me whenever they need me. Prospects are pleasantly surprised to reach a live person when they call my office at 8pm on a holiday. I had a few calls today (Labor Day) and now have a few tours scheduled for tomorrow. I also carry an emergency pager for the rare occasions that I cannot answer the phone.
Prospects ask me why my apartments are more expensive than my neighbors. I tell them "There's a reason the rent is $100 less next door." and they realize why.