MB's work fine and if you don't see your self going to other parks, cruises I would stick to what you have. If however you see your self going on a cruise, Disneyland and or Disney world then the MB+ will work at all of them. I really enjoy having mine and have used it multiple times at different places. My kids love the bounty hunting in star wars and the 50th anniversary statues in Disney World. The band would not be worth the cost for those but was worth the cost to be able to use it at multiple parks/ cruises.
The last four I bought from eBay. There were more options that I didn't see from Disney. They were also cheaper. Just make sure it says it has never been activated. I have never had an issue with any of mine.
Same! I have found some fun bands on eBay. As @lsutigers03 says, make sure they are new and not linked and also check the seller's ratings. If very few or poor ratings, move on.