Slot Machine is universally regarded as one of the best ices ever printed. It's extremely efficient and a nasty tempo hit. Against most common breakers, this usually causes a 4-6 credit swing, which is excellent for a 3 cost ice. It was removed from standard play because it was so strong against every breaker that it made good ice look unplayable. It ALWAYS makes sense to rez this. It will always cover its own cost, and it may even generate enough revenue to pay for its rez as well.
As a point of reference, Diversion of Funds is potentially a game-winning move (a 9 credit swing for 2 actions). A Slot Machine by itself will probably make this something like a 4-5 credit swing for 2 actions. That's a hell of an impact for a 3-cost ice.
The first subroutine is ALWAYS live. It doesn't care about the results of the slot machine. It doesn't even NEED slot machine results, it's live even if Hunting Grounds has killed the slot draw. The second subroutine is usually live (I'd estimate 65-75% of the time, higher in decks heavy on events).
Its third subroutine is rarely live, but there are some IDs where being able to place 3 advancement tokens on an installed card is valuable. (E.g. if you're playing advanceable ice, if you're playing shell games, if you're on Eternal Tennin, if you have a City Work Project installed, if you have an agenda out that you're confident that they can't get to, etc). Just having a third subroutine, even if it rarely matters, helps harden the ice against ice destruction cards like Hippo and cutlery.
One final note, I think it's cool having corp cards interact with runner deck composition like this.