First off, sorry Marbo, you come close, but you just dont really cut it.
Yet I ask you now, is Mephiston really as bad as the naysayers claim? Is he really terribly overcosted?
Lets look at a hive tyrant. For about 300pts, you can get a flying MC with 5 attacks on the charge at str6, rerolling hits, and having 4 wounds with a 2+ save at t6, and unreliable psychic defense (SITW).
Same thing with a daemon prince. For a little over 200pts, you can have a monstrous creature, with 4 wounds, a 3+ save, and t6, AND a 5+ invul (which both meph and the tyrant lack) On the charge, you prince hits 5 times at str6, rerolling hits and wounds (assuming Warptime, for a combat prince.)
Then lets look at mephiston. For 250pts, you get an INFANTRY model, who gets cover saves from having his left toe in cover, has 6 attacks on the charge at str 6, a jump pack, fleet, rerolls hits and has potential str 10, AND has FIVE wounds with a 2+ save at t6, AND, to top it off, has a psychic hood.
What it really boils down to is that Meph IS in fact a hive tyrant, on an infantry base, who costs less than his namesake. He has a potential 24" charge range, possible str10, brings psy defense, and is really easy to hide behind anything really. A tiny wall will give him cover, and a rhino will obscure him entirely. Sure, he will die to concentrated ap fire. But what if you cant see him? Uh-oh. If he manages to contact one of your regular infantry units, guess what happens? They die. What happens when he contacts one of your vehicles? It dies. What happens when he contacts THSS, a TWC unit full of SS, or any good invul save deathstar? He dies. The trick is NOT to end up in combat with such units, which an experienced player can do most of the time.
When you play meph how he is played best, jumping out from behind cover and slaughtering an infantry unit, before jumping to another unit 24" away, you will see what he is capable of. DONT attack units with good invuls/lots of PF's. You WILL die, for no gain whatsoever. Meph is best bullying those infantry units and fire support that the rest of your army has trouble getting to. Sure, psy defense shuts him down pretty effectively, but he is still no slouch in combat. If your opponent didnt bring psy defense... oh golly, this match will be fun.
I find that when you consider Mephiston not as a 250pt character, but as a 150pt upgrade for a librarian, you realise what a bargain he really is. He's Chuck Norris, all for the cheap cheap price of 250pts.