I'm fine with PreFlight, but I like them better at PHL and BWI. BWI is very cheap (and have nice staff) and PHL has indoor option.
The issue with BOS PreFlight is the lack of indoor parking. For about the same price, Logan economy offers indoor option (yes, I know about the PreFlight coupon, but the coupon rate is no longer $15). If BOS PreFlight starts offering indoor parking or is substantially cheaper than economy parking, I could use it more.