I know many people dis him, but I love my MP Megatron! Yes, he can be a bit floppy in the transformation process. I actually just transformed him a few days ago for the fist time in over a year, and my goodness! But as long as he is cared for and not abused (which should be a no brainer for something that is more an adult collectable then it is a toy) he remains relatively solid in both modes once transformation is complete.
And as far as proper G1 Megatrons (which, to me, means ones with a gun-mode -- show me the episode in the original cartoon where he transformed into a tank or a jet, and I'll revise my views accordingly) he is the best looking one you can get.

If they made a new gun-mode G1 Megatron, that was a bit more solid? And came with the stock and barrel extensions? (The ones on mine in the photos above are third-party.) And might be more in line with the new Masterpiece scale? Sure, I'd be on board. But that won't stop me from loving my original Masterpiece Megatron.
And as for scale... Well, I was watching an early season one episode recently. (Don't recall which one exactly, but it was on disc one.) And there was a scene where Megatron was towering over Soundwave. So... looks like the scale isn't as off as people complain about!
Yeah... I know... There's probably just as many, if not more, examples of them being the same size... or even of Soundwave being taller! Scale was never a consistnt issue on the show, or in G1 toys (where Soundwave was bigger then Megatron) so as long as we don't have a case of Megatron being two feet tall, and Soundwave is only two inches tall, then I'm fine with it!If you do buy the one you're looking at on eBay, good luck with the purchase, and I hope you enjoy the figure!