First, I have to ask: is this with or without IS? There've been lots of versions of the 400 f/2.8 over the years, and not all have had IS. That's mostly of interest to know whether or not the price is fair.
That writ, even the oldest ones had phenomenal image quality. Of course, it's evolved from phenomenal to magical over the years, but you're certainly not going to be disappointed nor missing out by going with an older model.
What the latest version primarily brought, aside from the higher price tag, is a significantly reduced weight. You can actually hand-hold the new version, though not for a long time. Not so with any other 400 f/2.8, period (unless you're Arnold Schwarzenegger). While you wouldn't want to get the new version with the thought of hand-holding it, it makes a huge difference when hauling it all over the place.
(It's also even better optically and has better IS, but that's just gilding the lilly. The older versions were as close to perfection in those regards as makes no difference.)
But people have been shooting with lead-baloon 400 f/2.8s for as long as there have been 400 f/2.8s, so it's not like it's something that can't be dealt with. The new version is absolutely worth the extra money, but that value is meaningless unless the money is in the budget to consider it as an option in the first place.