I've been boiling 7 gallons on the stove, and am considering buying a propane burner and moving to the garage. I think I'm going to go for the Blichmann, but buying a new burner is definitely a different question than replacing one. So, here are my questions to you to to help you make your decision:
1. How long does it take you to get your batch from room temp to boil? A higher BTU burner will make that faster, but is that a problem you really have? If half of your brew day is waiting for water or wort to heat up like mine currently is, it could be well worth it.
2. How much propane does your burner use? I haven't seen anything quantifying this, but from what I understand, the Blichmann is pretty efficient in it's fuel use. If you use several tanks of propane a year on your burner, that may add up to savings, but that seems somewhat unlikely to me. In combination with some of the other considerations, it could tip the scale though.
3. How clean is your burner? From what I've read, some cheaper burners turn the bottom of the pot black. Does this happen to you? If so, how much time do you spend cleaning it?
4. Do you just want it because it looks good? This sounds derogatory, but it really isn't. If your homebrew setup looks like something that would fit in at a junkyard, and you take up your garage or patio with it once a month, you and the people you live with and near might appreciate something that fits in a bit better.
Those are four areas that I understand the Blichmann to have advantages, whether or not they make a difference to you is your call.