Depends on the brand. There's loads of them.
Classic Army M24 is a clone of the ye olde PDi M24 that came out in the 90s. It's APS2 based IIRC,however the trigger mechanism is unique unfortunately. Quality wise it's pretty decent,stock internals are pretty crappy as per usual with sniper rifles- Hop up units are usable but need upgrading. Externals are very nice as per usual with Classic Army. Price wise it's not bad either. It's a capable platform,but not the best by any means. PDi make a fair amount of parts for these(They are guys CA copied after all)
Snow Wolf make two M24s- One that's VSR based and one APS2 based. The VSR based one has tolerance and dimension issues which means upgrading can be a pain and you might need to replace parts which you usually leave alone,like cylinders. The APS2 based one is a little better in that regard. APSones take APS2 Cylinder internals i.e. piston, spring and spring guide. Other parts such as trigger and cylinders are unique. Hop up units are complete shit on both,need replacement.
A&K S24 is probably the best one about. Comes with a 90 degree steel trigger mech stock,which is insane value for money. Piston is machined aluminium and since it's a 90 degree trigger there are no problems about reliability. Spring guide is also made out of steel,as is the cylinder. Trigger and cylinder are VSR-10 compatible,however these are some of the best stock internals around.
Hop up units are unique- Off spec to VSR version. However they take VSR 10 rubbers,barrels and hop up arms IIRC. You can modify the units by using spacers made out of brass shim stock to tighten up the tolerances. The whole gun is insane value for money.