At first glance, Fensirs seem like relatively straightforward beatsticks. They have four giant RHD, large size, a reasonable amount of strength and a smaller boost to constitution, +6 natural armor, and of course the rock throwing and catching we're used to from giants. They get a few odd magic abilities: the power to monitor the status of their identical twin, at-will Transmute Mud To Rock, and at-will Transmute Rock To Mud, but all in all they are fairly typical bulky beatsticks.
Except they also have 5th-level wizard casting.
This creates a difficult situation. At +1 LA, a fensir is still bulkier, more versatile, and much stronger than a 5th-level wizard, while having essentially the same intelligence. Reasons to pick the wizard are small, and on the whole the fensir is superior.
A +2 LA would put fensirs slightly behind in casting and feats (while still having better bulk), which might be enough to balance them out? They still seem like a strong choice for gish builds, though.
For now, I'll go with +2, though the issues of rating what's essentially an ogre/wizard gestalt aren't easily resolved.
Rakkas are fensirs who, in what's undoubtedly a smart evolutionary move (or more likely, the whim of a Chaotic god), have turned enormous, ravenous, and nearly insentient. Huge size is neat, and the better strength and constitution (and wisdom, somehow) are notable as well, but 12 HD and the loss of casting hurt a lot.
Aside from the chassis upgrade and casting downgrade, rakka only really distinguish themselves by having the ability to place a Dying Curse (essentially a Geas) on those responsible for killing it. Because this involves dying, I can't really recommend it to rakka PCs.
-0 LA: there's better giants out there, and most of them actually get to have skill points.