Much like Typhus, Fabius Bile's interesting quality comes not from himself, but from how he can affect other units in the game. Specifically, he is able to "upgrade" one unit of regular chaos space marines for free to enhanced warriors that have +1S and Fearless.
Now, why is this good? Well, one of the main problems with Chaos Space Marines is their lack of "And They Shall Know No Fear" (ATSKNF). They do fear things. And with a lower leadship value and lack of ATSKNF, there is a good probability that they will be routed off the board. Fabius Biles ability circumvents this without having to run cult units as troops (e.g. plague marines are fearless, etc.). If the troops we select are getting +1S, then we'd better tool them up for some close combat. So we're probably looking at a Mark of Khorne, with close combat blades galore, give the aspiring champion a power blade and even the odd gift of mutation for fun. Roll them up in a rhino, or perhaps even infiltrate them through a good warlord trait (or perhaps we've got Huron or Ahriman as a second HQ selection). Give them an icon of wrath perhaps, flamers and meltas to taste and watch what happens. Sure, there are other builds (Nurgle T=5 and S=5 marines are pretty scary), but they cost proportionally more. Equally, they could be exactly what a Fabius Bile -centric army would be all about: a large blob of modestly costed hurt (e.g. 10 marines with Mark of Nurgle cost 170 points, which isn't too bas at all!).
As for Fabius Bile himself, once the upgrade has been put in place, he should probably be placed in a second squad of marine to give them all fearless and charged in to the heart of a battle (or perhaps even used as fearless objective holders). In a fight, he's okay (he's not Kharn, you understand), but his feel no pain should keep him alive a little while longer than the average grunt and his ability to inflict instant death is nothing to sneeze at. The Xyclos Needler is also not a bad ranged weapon (18" range with assault 3 and poison) that can have a place in a close combat squad and is useful for the odd snap shot that may be got off in case a unit is charged.
In short, I think we're taking Fabius Bile as a utility choice for the upgrade to circumvent the lack of fearlessness in our regular troops. He's okay, but not brilliant, and appropriately costed I think.