FFG has a bunch of ways to weaken too strong ice such as making it unique, making it cost 5 influence and making it expensive to rez. Guess what, that's what Loki is: unique expensive HB-only ice.
It'd better be worth it if it's to see any play at all. And it is.
Loki is bioroid ice that can't be clicked through, meaning it can be used with all the bioroid support cards of HB without having any of the typical downsides. There's one sub that may as well say "End the run" as most runners will only go empty grip in a make-or-break run. But because of the wording Endless Hunger doesn't work on Loki.
Of course the real value of Loki is in the on encounter ability. Point to a rezzed piece of ice and Loki the subtypes of that ice but more importantly the subs.
I guess everyone first thinks of mythic ice, but I see more practical value in bioroids. Bioroids tend to be efficient but porous due to the runner being able to click through them. But the text that allows the runner to click through subs is located outside the subroutines. Thus if Loki copies the subs the runner will not be able to click through them.
The big problem however is that Loki has strength 3, which is easy to break with most breakers. However consider a Spiderweb. That's a barrier with 3 subs and strength 2. If Loki copies the subs it becomes a 4 sub with strength 3 barrier of which the runner needs to break 3 subs and wants to break 4 subs. A 3 strength 4 sub barrier costs 4 to break with Paperclip and 5 with Corroder. Other good barriers are Battlement and Eli 1.0 / Eli 2.0.
In the sentry department we have Cobra, Errand Boy, Ichi 1.0, Negotiator, Pup, Rototurret, Shadow, Swordsman (though it may be better when encountered directly if the runner has an AI), Tsurugi, Veritas, Vikram 1.0, Zed 1.0/Zed 2.0 (if Loki is behind click-through ice).
And for code gates there are Aiki, Authenticator, Bloom, Datapike, Enigma, Fairchild 1.0/Fairchild 2.0, Harvester, Herald, Hourglass, Hudson 1.0, Inazuma, Interrupt 0, Macrophage, Ravana 1.0 (if you really want to go crazy), Viktor 1.0, Weir, Yagura.
Of course there is many more ice and there's plenty of argument why some ice combos well with Loki and other doesn't. The ice I have highlighted is roughly ice that's either cheap or medium cost in combination with an annoying restriction that makes it work better on Loki despite the low strength and that in any case has multiple subs. I've excluded some ice like Archer which is very costly to rez (i.e. forfeit agenda or bad publicity). By no means is this list exhaustive however and I look forward to suggestions about other cards that work with Loki.
Because Loki works so well with a lot of cards it's very flexible. If you have a barrier, sentry and code gate rezzed you can determine during a run what would be most taxing for the runner. For example if you know you have a code gate behind Loki and the runner has a Gordian Blade you may decide to make Loki a barrier. Or if you see the runner has only a trash-to-use killer you may want to make Loki a sentry to get rid of that killer.
It's this flexibility and the potential for being a 3 or 4 sub ice that makes Loki worthwhile.
Because of the low strength Loki is vulnerable to ice destruction however, so keep that in mind.
Oh and if you happen to run a HB Sandburg deck Loki should fit in quite well.
EDIT: A previous version of this review said Hive works great with Loki because the subs don't go away when used on Loki. This is incorrect. Thanks Toper for pointing that out.