Belial in my opinion is perhaps one of the best characters in Codex:Dark Angels.  While he still suffers from nerfed stat lines, he simply delivers more bang for your buck than almost any other character. 

With Belial, you effectively get not just a Company master (that would normally cost 100 points), but you also get a master-crafted power weapon and storm bolter and Terminator Armor for just 30 points.  Of course, you lose the iron halo of the Company Master with Belial, but you still get a 5+ invulnerable save because of the terminator armor.  With 6th Edition, this loss in invulnerable save is actually not nearly as bad as it was in 5th edition, as Belial has become much more survivable.  For one, power weapons became nerfed, and Belial's Terminator armor will still provide him protection against most of the power weapons that he will encounter.  Really, the main things that He needs to worry about in assaults are unwieldly weapons and some special weapons, but even these are at a disadvantage as Belial will be able to attack before they get their chance.  Also, with the boost in Feel No Pain providing protection against everything but instant death, and with the new Look Out Sir! rules, Belial has become an easy choice to include alongside a command squad instead of a Company Master.

We also have to look at Belial's options.  If you want an assault beast, you can give Belial lightning claws giving him one more attack and allowing him to re-roll failed wounds.  If you want more of a tank and anti-armor specialist, Belial can also take the Thunderhammer/Storm Shield combo giving him a very nice Invulnerable save at the cost of not attacking at initiative.  I haven't fully decided how I plan on fielding Belial just yet, but I think I am more leaning towards the Lightning claws as the lists I have been building lately already provide more than enough anti-armor punch, and the loss of two storm bolter shots isn't a big deal at all.  Although, the Storm Shield may become the more favored choice simply for the protection it provides.

Belial also provides a couple of other big benefits to the army.  Perhaps most importantly, he makes terminators scoring units.  This could be quite handy in most of the game types, as a unit of terminators is a very hard unit to knock off of an objective.  And secondly, Belial effectively allows you take a terminator command squad.  This in many ways is better than a normal command squad not just for the added protection, but also for the fact that it is a scoring unit as well.  The only draw back to this "Command Squad" is the price, but if you are going to include terminators in the army anyways, this becomes a very viable choice.  Regardless, in my opinion Belial is still a very good choice for any army, even if you decide not to include terminators.