Spoiler (click to reveal)
I find the new squadmate entertaining. I took him to Sur'Kesh. When Wrex threatened to eat the Salarian soldier's livers raw if they didn't comply with his wishes, Javik commented something to the effect of "Wrex is correct. In my time, that was a delicacy." Awkward silence ensued. I can imagine Shepard taking him aside later and trying to explain some of the finer points of inter-species diplomacy, like don't casually acknowledge you've eaten some of the other species, for example. It's just not cool.
The Protheans are precisely what I didn't expect them to be, which is perfect, and exactly what they should be. I've seen a number of sci-fi games with something like the Protheans - advanced progenitor races who mysteriously vanish. MoO had it, Star Control 2 had it, GalCiv II had it... Seems a common trope. In every other context I can think of, these races are portrayed as morally perfect, idyllic, and near omnipotent, but invariably missing for some mysterious reason. The Protheans, as far as I can tell, were jerks. Not evil, per se, but not really people you'd want to share a galaxy with. Anything closer to type would've been eyeroll-worthy, but I think what they've done works incredibly well. Given how idealized the Protheans are in the ME universe, Javik is hilarious, and is currently probably my favourite squadmate to bring along simply for his dialogue.