You can play Dungeon Runners for free, with no limit to time or levels. You can go sign up right now, play until the level cap, then start a new account and do it again.
Your $4.99 per month buys you the full features. You cannot equip the top three tiers of items unless you are a paying member, meaning you will have worse damage and defenses. Members can use the good potions that stack in inventory, meaning you get more pack space. Only members can use the bank to hold items. If there are queues, members go to the front of the line.
Is it worth it? There is no box to buy, so your cost really is just $5 for each month you play. If you played Diablo II for less than 6 months, you would have been better off with this. Also, you can play for a month, leave when you get bored, play another month after a few dungeons are added, and so on. It might take you years to get through six months, over which time there will be the equivalent of several free expansion packs.
It is a free download with unlimited trial, so give it a shot. Play through the newbie area until you get to Townston. If that was fun, play until level 10, to see if the gameplay is sphexish. If you are still having fun, throw them $5 and start using the good items. It was much better entertainment than I got from that matinee of Terminator 3.