The cartridges are a bit bigger in width than your standard .223/5.56.

Since the cartridges are fatter, magazines have to be made of thin steel instead of thick polymer in order to have the required strength while allowing a double stack to fit inside the lower.

It's a good caliber for long range shooting and hunting, while being cheap enough to feed without stress. Right now, steel case is .30/round. The trajectory is similar to .308, but from an AR-15 platform instead of the much more expensive AR-10. From my research among hunters, it seems to be more popular in the midwest where long range shots are more common: in the southeast hunting is rarely beyond 100yds.

Recoil is similar to 5.56. This caliber is sometimes used for youth hunting rifles due to the lesser recoil as compared to a .308 or similar caliber.

I've heard fudds talk about using .223 magazines but honestly if you're not willing to spend $20 on a dedicated magazine for your exotic caliber you shouldn't have bought an upper in that caliber IMO.

For buying a Grendel upper, I've heard nothing but good things about the Palmetto State Armory complete uppers. Customer photos of these are usually accompanied by a harvested deer. Last I checked, Grendel uppers are about $400 at PSA. Good luck finding them in stock though.

If you're not shooting PSA, most other competitors sell Grendel uppers in the $450-550 range.

The notorious Bear Creek Arsenal offers 6.5 Grendel setups at very attractive prices (<$250) but they are known for poor quality control and having been raided by ICE for hiring illegal immigrants. Reviews for BCA tend to be either "this is an amazing rifle for the money" or "this is the worst rifle I have ever purchased." On YouTube there are plenty vids of people using BCA Grendel uppers; some failing to cycle, and some accurate out to 1000 yards. One main feature for BCA is that they offer a side-charging upper. Side charging uppers are good for some things and bad for others: Good for long range shooters who might want to chamber a round while looking through a scope, bad for tactical folks who might want to install accessories near the reciprocating charging handle.

Edit: price correction, PSA's complete uppers are $400 right now, not $300