You can get loads of rechargeable batteries on Amazon for the same price as one decent set. And the temptation may be there, especially if you're running multiple speedlites etc. But should you?
Over the years I've leant "buy cheap, buy twice", but with new companies constantly coming out with decent gear for a fraction of the price, it makes that saying a little redundant.
I originally bought a set of PowerEx batteries which cost nearly £20 to put in my flash, a midrange set, and 6 sets of cheapo ones from Amazon. I was reluctant to end up paying near £100 for literally 5 sets of PowerEx ones. I soon learnt the difference between them.
The cheapest ones are rated at 2900 mAh, PowerEx are rated at 2700 mAh and the third set are rated at 2650 mAh. So you'd think the cheapest would hold the most juice. Nope.
These cheapo ones were ok for around 3 charges, but would take an absolute age to get to capacity. They also depleted crazy fast... Not ideal. Plus, they older they got, the less maximum charge they held and faster they'd die. Pretty much useless.
The PowerEx batteries on the other hand, were strong from the beginning. Kind of what you'd expect from the price of them. They also charge pretty fast too! In a speedlite, these batteries are supposed to go for 100 full power flashes too! Crazy right! Recycle rate (time it takes to pop the flash again) is impressive too. Especially with a full charge and lower flash power. I've used them for full days of wedding shooting multiple times with no issue.
For me, power retention was always an issue. So if I'd charge the cheapo batteries a couple days before a shoot, they'd already be dead. Again, not ideal. The PowerEx batteries wasn't perfect either, but I'd estimate they still would retain around 90% of their full charge.
The price difference at first seems huge, but if you factor in the amount of cheapo batteries you'd have to buy to keep up with a decent set it becomes a no brainer. I really wouldn't trust cheapo batteries in a shoot, let alone a professional job.
*Note: I barely mentioned the midrange priced ones as they were exactly as you'd expect, in the middle of both.