People who are still playing this game at least a year after they bought it should automatically mean that it's worth the $20 to them.
No, it's worth the 20$ if you like it. I don't extensively play BL at all; it's a great game in concept but lacking in mechanics, in my opinion. A gridlocked building system is limited and the eventing system is rudimentary at best. Cars are nice, but the vehicle physics blow more stuffs than I can count on two hands.
You could say Minecraft is the same thing, but I don't think that's worth 25$ either. Both games are fun, just not ranking very high on my preferences.
EDIT: And yeah, I should probably mention that my entire point was to actually say that a lot of people wouldn't buy this for 20$, but for different reasons.