I really like my partner in this game.
The last dungeon I completed was Brine Cave. I would've been kicked out of that dungeon ten times over if it hadn't been for Cherri. Floor upon floor of Water Pokemon, finished off with a nice helping of Kabutops and Omastar-style punishment. Since I'm playing as Cyndaquil, that dungeon was death. I was glad to have my partner there, as she could take Water-type damage a lot more easily than I could, and deal with the offenders much more quickly.
It wasn't only that dungeon - Cherri is a big help in most dungeons. Her Solarbeam attack is very powerful, and she can take a lot of damage. There's just the issue of trying to get her positioned near the enemy when I need her to defend me.
One thing that annoys me, though, is when my partner or one of the other team members somehow gets separated from the group. If I have a Rollcall Orb or I'm close to the stairs, it's not too bad, but if I'm nowhere near the stairs, it's a pain to have to be interrupted every step with a scene of my partner/another group member trying to fight a wild Pokemon.
An especially bad example of this was when I had to escort this Level 7 Weedle through Amp Plains. The idiot had gotten itself separated from the group, I had used up all my Rollcall Orbs, I was nowhere near the stairs, and I wasn't able to assign tactics to client Pokemon (preventing me from telling it to wait in one spot). Of course, the moron got itself trapped between a bunch of wild Pokemon and was hopelessly outclassed. By the time I had finally tracked down that Weedle, it had used up every single Reviver Seed in my bag. It took me a long time to replenish that supply afterwards.