Bullet Seed - usable?

Yes it is of course a grass move so for pokemon like rhydon it is better and for others weak to grass it is only a little bit worse than extrasensory.

Sure is...it builds energy fast, and some prefer dbl grass moves.
For Rhydon, and even Vapes it will do very well - for things like Blissey and Snorlax then many would prefer Es, but that doesn't stop it from being a good move.

Try using it and see if you like it, before making your decision.
Personally, I prefer Es for it's versatility and dps but there are many out there that prefer Bs...so it comes down to preference more than anything else.

Definitely useable. Bullet seed isn't as powerful as ES, but it'll get the job done EASY against Vape and Rhydon. Also good against lower Blisseys (2700 or so)

Yes, against Pokémon that have double weaknesses to grass.

Bullet Seed has better DPS and a bit faster charge move output than extrasensory, if the opponent has double grass weakness.

A good trainer has exeggutors of both skill sets.

This is my favorite moveset for attack, because I'm only using my Exes versus Vaporeon or Rhydon.

Definitely usable and in fact, very good against those weak to grasses. Not too far behind ZH/SoB against Blisseys too.

https://youtu.be/IV8Zdp679pI

Just to reiterate what every other comment has mentioned other comment has mentioned: BS has a slightly less lower dps against mons that take neutral damage, but the energy gain is higher meaning that it will be able to fire off solar beam faster, which compensates for the lack of damage output from the attack itself.

And the fact that it tears up vape and even more so rhydon