If you have enough (abundant) credits I'd say go for it. It's an unusual ECR and who knows when such an offer will happen again. Onyx is one of the worse legendary mechs, but getting it for cheap and maybe going through its achievements could be worthwhile. Plus if it ever gets buffed in the future you might regret not picking it up now :-).

It's not a good replacement for anything in your main hangar though, even at 5 stars. I wouldn't bother upgrading it.

Onyx is one of the worst legendary and it needs a buff, but if plarium actually buff it you should get it just in case

I got Onyx in the Crate Rush and its still Trash. Choose Hemlock its very funny to stun enemies from the other side of the map

Sure the Ability from Onyx IS cool but weak

Well onyx is crazy in hangar showdown seriously. I play at manual aim and league 2. When I perfectly use it's ability it deals 140k damage. 

Onyx is weak in competitive matches cuz it has low HP and Speed, most ppl (including me) consider Onyx as an easy target (It reminds me of Brickhouse ECR, because many players using Brickhouse would be killed by my Surge+Revokers). In HS mode, it's usable to some extent and can defeat Epic mechs with low HP pretty easily. 

Onyx Needs to be much tankier and also, unlike other mechs, it does not have a modifier to improve it's ability DMG/ have a reduced cooldown. Onyx certainly needs a HP buff which can solve most of complications