I bought a "genuine" battery from Trade Me and it was counterfeit (the text was a slightly different font, but the proof was when System Profiler reported a different serial number than the one on the actual battery). I was able to return it, and I picked up a third-party one from iFixit instead. I'm much more inclined to trust one that freely admits to being third-party, as opposed to a counterfeit!

That was three or four years ago and it's still working.

the ifixit ones arent cheap either. to me as long as it delivers constant rails and amps im ok with it. most trademe sellers have good ratings.

hard to know which one is a good cheap knock off though.

but makes nos ense to spend $300 on a $300 notebook

I got one for my trusty old HP DV3 from Hong Kong (I think) as my old one wasn't holding a charge well. Connected it up and it wouldn't charge. Contacted them and they duly sent me another one - with the same problem. It was going to cost more to return them than what the battery itself cost, so I said stuff it, bit the bullet and bought a genuine HP battery, which worked absolutely fine.

yeah dont have that problem with the trademe ones.

the ones on Ebay that say Genuine Mac as cos $80 and have all the identical macbook printing on it make me giggle, if they were genuine why they so cheap.

but regardless, if this was a $1500 notebook i might consider a $300 battery. this is Apple$ remember !

One for my HP DV6 and one for an Acer R3 to replace this...

Both work fine. 

two years ago I bought a battery for my Thinkpad off Aliexpress for about $30. 
Couldn't tell the difference between this and a genuine part. Still going strong.

3rd party batteries can be a real lottery. Personally I prefer to get genuine for Windows laptops and phones, especially anything lithium, but if you're talking about some horrendously expensive Apple thing, (SAVING $300 on a battery? What the hell? What does it normally cost?) I think I would go compatible too...

Ive bought clones for old dells that I have around the place as expendable computers for things like messing with arduinos where I might blow it up thru the USB etc.

All have been fine. About $35 each for them. When one died eventually I opened it up hoping to find 4 working 18650s as is usually the case in a "dead" laptop battery, and instead it had 6 pouch type lipos inside it. Never knew that, it worked fine all along till it was left flat too long.