I was afraid to use it because if I put a high level Pokemon in and everyone else is ripping each other off with low level ones, then I'll get ripped off even worse, but if I put a low level Pokemon in expecting to get ripped off and I get a high level one, I'll feel awful...
Level is not quite so important in Wonder Trade, what makes 'good' Wonder Trades are usually;
- Rare (it's extremely unlikely but possible you may get a Xerneas/Yvetal from someone resetting their game to play through again if they cycle their collection through WT before reset; more likely are later-game or hard to find 'mons, like Kangaskhan, Ferroseed, Dratini etc., or starters - Kanto or Kalos - that you didn't pick)
- Hidden Ability (not everyone has a Friend Safari available with every possible species yet, so getting some decent HA pokémon is nice)
- Version Exclusives (Tyranitar/Aggron, Houndour/Electrike, and a couple others I think - you could snag them off GTS easily but landing them through a random WT saves you the trouble)
- Good breeds / catches (2+ 'perfect' IVs; Friend Safari pokémon always have minimum 2 perfect, unbreedable pokémon - including 'baby forms' like Azurill, which can breed when they evolve - always have minimum 3 perfect when caught, and obviously breeding and/or random luck can provide more. Good breeds can also come with egg moves, which are otherwise time-consuming to breed for)
- Vivillon (if a butterfly collector)
- Pokérus (the 'live' infection lasts 2? days, or indefinitely if boxed. Doubles EV gain, which in turn halves the time needed to train pokémon)
So you'll usually get pokémon at level 1 (just bred), level 30 (just caught in Friend Safari), or something else (usually, just caught in the wild). I've been sending out bred pokémon at level 1 with 3-5 perfect IVs and a selection of appropriate egg moves, because I love the breeding side of pokémon (...not like that), but am hopeless at team-building and similar things. Maybe this gen, who knows.
To be honest, though, putting pokémon into Wonder Trade isn't likely to get you awesome in return; it's purely a gamble. Worst case scenario you have a different pokémon taking up box space; best case, you've got a good new team member, pokédex entry, maybe even a new item if you're lucky.
My own policy is to only put 'good' pokémon into WT and either release the 'useless' ones or keep them for use with the Loto-ID, but my definition of 'good' is subjective; someone who doesn't know about IVs, or the use of entry hazards in competitive play, or egg moves, might have taken a Ferroseed that I sent out and thought "WTF is this, it doesn't even have an attacking move and it's level 1?!".