As mentioned above, in terms of value PSA, BGS, and CGC are the main three grading companies for Pokémon right now (primarily PSA and higher graded BGS actually). Does that mean cards graded by other grading companies are worthless? Definitely not. The SGC-10 1st edition Charizard that sold for a record 369k USD should be enough prove for that.
But as you can see in the last few comments of this thread of known grading companies I created, a lot of new grading companies have popped up these last few years. Although it might be fun trying them out, in terms of value and risk I would stick with those that are established and/or exist for at least three years (preferably 10+ years).
On a personal note, I’ve recently tried out PCA with a French Moltres. It’s currently on its way back from grading (took ~4 months, so not too bad imo) - I got a 9.5 btw. I actually already own three PCA-graded Pikachu which were a special French release authenticated by PCA, and which would be worthless outside of the slabs. After seeing their slabs in person, I kinda liked them, so I asked a French friend of mine to send a NM French Moltres to see how it goes. I’m not sure yet whether I’ll do the same with an Italian Moltres to GRAAD, a Spanish Moltres to EG, and a German/Portuguese Moltres to an established German/Portuguese grading company (if there even is a Portuguese grading company… 
), etc. For now, probably not.
As for PCA, @mareee posted some pros and cons and his experience grading with them here: www.elitefourum.com/t/pca-grade-graded-pokemon-cards/24169/1
Greetz,
Quuador