I had this conversation with a friend of mine from church last month. She had a new unit installed 10 years ago and bought the additional labor warranty. She said it was some of the best money she had ever spent. A problem occurred with her unit right before the 10 years was up and she didn't have to pay anything for the repair.
In my opinion, $55/year for that kind of peace of mind is well worth it. Not to mention it's quite possible you could recoup the entire value with only one repair. I would ask myself this question: Will I regret spending this money more if after 10 years, I never used the warranty, or if within 10 years, I need a repair (or more than 1 repair) that costs me more than it would have cost for the warranty? My answer, in this case, would be to spend the money on the warranty, but I will admit I tend to over-insure.
Three things that might sway your decision one way or the other though: I'm assuming this is a manufacturer's warranty, so the warranty goes with the unit, not the homeowner. 1) This could be a good selling point if you ever need to sell your house. 2) Do you feel comfortable that the manufacturer will be around for the duration of your warranty? 3) You usually have to pay to have your unit serviced twice a year in order to keep your warranty active. This can be quite costly when they have to clean the coils or do other "maintenance" to keep the warranty in effect.
Whatever you decide, I'm sure you will enjoy your new AC! We're without AC at present, so I'll be sitting here in front of my fan, being jealous. :-)LOL
Blessings,
N.