Back during the dot com days the joke was the only motivational poster that did anything was the whiteboard that had the current stock price on it.

I don't think I've ever heard someone say "Wow I really like the motivational posters" but I have seen some that went pretty wrong (and as a result were ineffective).

So my guess is the have high downside and little upside.

That said, nice artwork (no pithy saying), seems to always make the space feel more inviting and comfortable.