Bought a Sealey long ratchet, £20 new. (this was the 300ish mm, admittedly) played with it a bit. Nasty 'tang-tang-tang' noise from the mechanism. Bought a Halfords long ratchet - £25, has a flexi head, the mechanism feels so much nicer, better finish all round, comes with a lifetime guarantee, too.
Appreciate this isn't helping you on a 600mm kind of length you're looking at, however, I have to say, once the breaker bar has done it's work, just 'cracking' the bolt/nut, I've always defaulted back to a 'normal' length ratchet with no real issue. This Halfords with the flexi head having that extra couple of inch over standard is the best ratchet I have used, and is spot on. Frankly any longer and it'd be annoying/awkward to use (imagine inside a wheel arch for instance)
Of course Mr The Bear you have to use what you want to use, but personally between my ratchets...
-standard 3/8 drive,
-the 1/2 drive (that bit longer, but twice the weight, AND annoying when you're on your back and it slips, and falls on your face!)
-my 3/8 drive long flexi-head
I've no feel of any need for an even longer ratchet.
Got the longest Breaker bar Halfords had - just over £20 as I recall, and love it. As I say, a couple of cranks on that and I'm straight on anything with the flexihead ratchet without ever feeling 'I wish I had a longer ratchet for this bit'
Can second what Nick W says, too. Used my flexi-head on turning the crank, on my back, and that extra leverage helped no end with that.