Versatile offers every investigator the ability to buy any one level 0 card at the substantial cost of 2 experience and an increase in deck size. I actually think the +5 Deck Size is the greater cost associated with Versatile than the experience because its going to make it harder to find and draw your key assets - weapons to fight with, tools to gather clues with, etc. - not to mention also making it harder to find that special out of faction card you bought Versatile to play in the first place! I don't think its worth buying Versatile without an exceptionally good reason; the out of faction card you buy better offer you something especially powerful for your investigator. There are a hanful of investigators that get something really unique from Versatile or else who I believe are in the best position to take advantage of it:
Wendy Adams - Wendy gains access to Premonition, which combined with her amulet truly breaks Arkham Horror. The Forced ability on Wendy's Amulet is presently ruled to not apply to events that don't go immediately to the discard pile after being played. This means that, with Wendy's Amulet in play, you can play Premonition over and over again at the player window at the start of every skill test and always know exactly which token will be drawn. To dig to find this combo, Wendy has access to Rabbit's Foot, Drawing Thin, and Nothing Left to Lose, among other cards. You still won't reliably find your exact two cards in 35, and the experience cost of the best Survivor draw makes this combo likely a late campaign option at best. Personally, I think infinite Premonitions will spoil the major challenge of Arkham Horror - in having to guess which skill checks are worth your resources, and sometimes being screwed by bad luck you couldn't foresee.
Mandy Thompson - Mandy is a special case on this list. Her sheer power of deck searching means she cares far less about her deck being a bit on the large side. If you are playing Mandy with a base 30 Deck Size and all the generic deck searching cards that are so strong for her - Mr. "Rook", Eureka!, etc. - then you likely will find the card you bought Versatile for in most scenarios. The question remains as to what you should buy. Given Mandy's insane searching I don't think you need as powerful a combo as other investigators considering Versatile, but I'd suggest a powerful out of faction asset to make up for the fact she only gets events and skills in her "off-class".
William Yorick - being allowed to replay assets means that any you buy will get value for as many times as you can afford to play them. In addition to that though, Occult Lexicon offers Yorick a truly fantastic ability with Blood-Rite. Blood-Rite lets you discard assets you'd rather play actionlessly from your discard pile in exchange for the two things Yorick wants most of all: resources and damage. On top of that, Yorick's card pool contains a broad variety of powerful weapons all of which he's happy to play - Enchanted Blade, Baseball Bat, Meat Cleaver, etc. - so you can keep the desity of weapons in your deck high even if you add 5 more cards. In addition to the powerful Survivor draw available to Wendy, Yorick can use Yaotl to mill his own deck to dig for assets.
Calvin Wright - Calvin uniquely benefits from having his health and sanity pools extended because it means his stats cap out at higher values. The number of cards in the game that presently do that is exactly two: Five of Pentacles and Hawk-Eye Folding Camera. Five of Pentacles is already a Calvin staple. Is the camera worth it for 2xp and +5 Deck Size? The willpower and intellect boosts are also nice to double down on deepening his sanity pool. I'm not certain this is worth it, however. Both Five of Pentacles and the camera demand that you mulligan for them because they are far stronger than in your opening hand than drawn later in the scenario, and all the card draw and deck searching in the world won't help you mulligan your thick 35 card deck. If you want to give Calvin a camera, you might be better gifting it to him by the use of Teamwork.
I think these examples show well enough when Versatile is likely to be good. It's not really worth it if all you are doing is buying some generic value card from out of faction, it really has to be a card with a unique effect that is particularly powerful with your investigator that you wouldn't be able to substitute with in-faction cards, and even better if you have access to plenty of card draw to dig through your bloated deck to find your cards.