I just got one for the reasons mentioned above - (weight of heavy barrel vs. the A1 etc) - capacity out of the box is great for the size at 6+1 - S&J Hardware makes a great +1 extension which brings it flush with the 18.5" barrel ($55) - so it can be extended to 7+1, only down 1 round to the longer and heavier 20" barrels, impressive!
I got mine for $265 delivered to my FFL for transfer, using Buds Gun Shop "price match" tool - phenomenal! I'm very happy with it, haven't shot it yet, but its definitely light and nimble (I have a 12" Hogue LOP stock on it now, I'm upgrading to Magpul SGA stock soon)
Its lighter and more nimble with the polymer furniture than my M500 with 18.5" barrel and 5-shot tube, which sports wood furniture - not by much, but definitely a little.
I am with you on the nose heavy feel of the A1's, especially with 20" barrels - Since I always need a short stock because of my stature, it is the biggest factor on myself not getting a 20" barreled A1 -
I assume replacement mag springs are easily obtained from Mossberg, on the 6-shot tubes, as all the 18.5" 590 and 590A1 models now ship with the longer 6-shot tube, so that shouldn't be an issue.