The reply value is probably standard fare for a single player game. However there is kind of a Crackdown hint to it with the Riddler's Challenges(over 200 of them). There are things you have to find like the orbs, but they're puzzle in nature. Kind of requires a thinking cap since some of them you can see but you're not sure how about getting to them. Trying to be vague in order not to spoil. In theory you could play the whole game without doing them, but I find myself playing slower & more methodical so I can do these as I go through the levels.
As for solutions, it's not totally free-form, but it's not totally linear either. There is a map that will give you waypoints for your next task & you can go about whatever way you want to accomplish it. If you want, you could just ignore the that & go do whatever else you want to do(explore the rest of the island, do Riddler Challenges). What gadgets you have will determine how much you're able to access. Anyway, there may only be 2 to 3 ways of doing things & sometimes there might only be one way. Some you'll have to put a thinking cap on. Basically it's got a Splinter Cell mechanic to it. So you can go slow & stealthy, but then without the hand-to-hand combat you won't rack up combos to help with leveling up. There's definitely a balance that was intentional. I haven't played the demo so I'm not sure what level was included, but if it's the first level it won't give you much of an idea of how some of the better later levels work.
BTW, the other half of the game (non-story mode) are challenge maps. Think horde mode but as a brawler. It comes with quite a few of them, but then there are also some that came with pre-orders. Dem Bones & Crime Alley. PS3 gets the ability to play all of the maps as the Joker, but no exclusive maps were included. It's just another playable character. I think this part of the game is where the majority of the replay value will come from. There's been talk that Eidos might release other playable characters or different Batman suits as DLC. Though people have already found out how to mod the character skins & models on the PC and have started creating their own. Anyway, I haven't tried any of maps yet. I decided pay more attention to the story first.
Tommy McClain