He's one of the few Est-like units I actually do sometimes use. This is largely because Awakening plays so much differently than the other games, with such a lower emphasis on turns and time and efficiency, that on the few occasions I play the game, I go with a more relaxed state of play.
His status as an Est is up for debate if you're into that sorta thing. Ests are normally characterized by showing up late while he joins early, but chucking that out the window because arguing over the specifics of archetypes is somewhat silly at best, Donnel's performance can easily be described with an upward parabola.
He starts off low, soars to unseen massive heights, and then drops right back down to where he began.
See, Donnel joins you with bases that are so bad, I almost find them funny. Then his growths kick in, and even by Awakening standards these growths are outrageous. Throw a second seal on him by level 10 (or 15 if you want underdog I guess, for me, meh) and he'll probably be able to really carry his weight. Then he promotes, and - by god he is wrecking everything. Then you beat the game - at this I usually don't care too much about anything at this point, most the time I don't even get that far - and then the other half of his flaws kick in.
Donnel has mediocre max stat modifiers and the most limited class set in the game. He's not useless for the post game, in that everything outside of Apotheosis is still easily beatable by a non min-maxed team, but if you are playing with the hopes of min-maxing, he's terrible. Outside of passing down the pegasus knight and mercenary class sets, there's not too much to say about him.
I do like the explanation as to why he's so lopsided. At first he's a villager, not very skilled, no training whatsoever. Once he joins he is encouraged to try his hardest to match the rest of the army, prompting his sudden growth. But because he lacks formal training and experience that others do have, he hits the cap on his potential extremely fast.
Donnel, objectively terrible, fun to use. Both benefits and is harmed by how differently Awakening plays from most entries.