Solo play doesn't particularly dissapoint in my opinion, but it is a step down from multiplayer. Though I have only played with one other (Mrs. A) so far.
The funny thing is that the game itself plays pretty similarly with multiple players. There is no kind of scaling, so in part you could argue that it is less of a challenge. However, the interplay as you sit there trying to coordinate things and deal with new situations is pretty priceless. I'd say that single player is a tenser experience that will go well if you like carefully planned and executed play (single player as The Cleaner or The Lookout is probably most comparable to classic stealth gameplay). Multiplayer is a lot more hectic. Someone will screw something up and then there will be a crazy moment as the entire team tries to deal with it (it's very Three Stooges meets Reservior Dogs).
What strikes me about it is that it is very much a game. There's no real attempt at simulation here. The abstraction runs high and performs really well (e.g. you grab giant shiny coins to get extra uses of your equiped item). It makes me think of a kind of cooperative play I don't often feel I see in video games, reminding me almost more of co-op board games. As a matter of fact now that I think of it I am surprised the 'heist' concept hasn't been used for more video and board games.