I've run into a bunch of super mutants at level 3-4. The low level ones go down pretty easy (shotgun blast, a couple hunting rifle/chinese assault rifle hits). The brutes and higher level ones are pretty tough if you encounter them alone. The mini-gun brute was painful, though I did cripple his arm with my second shot to make him drop it (he picked it up again...). No deathclaws yet though, but fire ants and mirelurks are painful at times, though I am playing on Hard difficulty.
I've noticed a huuuge variation in the difficulty of enemies after a while, even ones that are the same (like raiders early on are much easier than the raiders later on with missile launchers and flamers). Don't trust the stuff you meet early on to be a good indication of what you'll find later. I took down some super mutants at the start pretty easily, later ran into a group of 3 brutes that nearly killed me a few times. At the moment the combat shotgun and chinese assault rifle are my best friend (haven't found sniper rifle yet), but the hunting rifle single shots a lot of lower level creatures when hidden.
Overall I like the game... I do have a ridiculous amount of ammo (300+ for laser pistol, 300+10mm AP, like 40 shotgun shells, ~130 .32mm, like 100 rounds for assault rifles which are .303 or something, flamer fuel, 20 magnum rounds... etc) at level 7ish and am in the process of clearing out a Talon squadron military base for the fun of it. So far the energy weapons I've found have been utterly un-impressive vs actual bullet counterparts, even considering my perc and agility and small guns and energy weapons are pretty comparable.
Killed my first behemoth... or really just ran around waiting for it die. The Arefu quest storyline was really bad IMO and was just really nonsensical, brought me back to Oblivion in terms of subject, a reminder which I didn't need.
I think I've used the +1 to SPECIAL skills perk 4 times now. My stats are pretty ridiculous. They should've capped that, because honestly, that's probably one of the most powerful perks you can get and its at level 2.