What exactly are yakshas?
Answered here: Who are Yaksha-s.
Is it good or bad to worship them? Will they get angry if you stop praying to them?
It's about personal liking. As mentioned in above link, Yaksha-s have attachive qualities like Raksha (also known as RAkshasa). Hence they are prayed in mode of passion (Rajas).
Upon stop worshipping, they will stop giving the results for whatever purpose were they being prayed.
My point is that should we worship Kubera?
Yaksha or Raksha necessarily don't mean "evil minded". They have attachive quality due to Rajas involved. Hence, those who have desires for anything, would worship in influence of Rajas.
If one acts desirous of wealth, then it's equivalent to worshipping Kubera.
BG 10.23 - ... among Yaksa-s and Raksha-s, I am lord of wealth (viz. Kubera) ...
What is worshipping?
Often people limit the worshipping only to "praying with hands folded and eyes closed". In reality, various worshipping ways are mentioned in Gita:
- dhyAna-yoga (prayers)
- chanting (japa)
- listening (shravana)
- kriya-yoga (Asana)
- karma-yoga (dedication)
- gyAna-yoga (illumination)
- service (seva)
- animosity (shatrutA)
- friendship (mitratA)
Whenever a person is desirous of wealth, he/she automatically worships Kubera, even if the person doesn't know the name. Similarly if person loves water bodies (river, ponds etc.), then he/she overall worships Varuna.
Whichever form one worships, the Self would fix one into that form. Though this way of worshipping is not methodical, it eventually leads to the supreme only.
BG 7.21 - Whichever form [of a deity], any devotee wants to worship with faith, I strengthen only that faith of them
BG 9.23 - O son of Kunti, Even those who being devoted to other deities and endowed with faith, worship [them], they also worship Me alone, following the wrong method.
Hence worshipping wealth (Kubera), eventually reaches to supreme. Eventually that would result in wealth. See this answer. Whatever attachive qualities come with money, one has to bear -- be it good or evil.