As @Greasythumb said, "Rex" is most useful in faction and might not worth the influence cost to splash it.

But, is it really useful for Criminals? This faction also have Peacock; not a great icebreaker, but in many way comparable to "Rex". Both cost 3, but Peacock have one more strength. Peacock cost 2 to boost strength by 3: so it's more efficient but less flexible than "Rex". But, Peacock can be used on the long term while "Rex" only have a maximum of four uses.

By exemple, passing an Enigma cost 4 with peacock vs only 1 and one counter with "rex". Tollbooth: 7 with peacock vs 4 and one counter with "Rex". Merlin : 4 to 8 with Peacock vs 3 + 1 or 2 counters with "Rex" (half of it's power counters!)

So yes, in faction, it's a good choice if you lack influence for something better. But you must have a plan for the long term. And please, don't waste influence to get shaper cards for "Rex" recursion when this influence could have bought a better decoder.