I've tried almost all of the available iPhone "RPGs". Most are pretty horrible.
The best of the games currently available is definitely The Quest. It feels very much like Daggerfall translated into a turn-based combat game, with a tile map and 90-degree-rotation and step movement. I have a lot of pleasant memories of the early Wizardries, Might and Magics, and Bard's Tales, as well as enjoying the more recent Etrian Odyssey games, so this format of presenting the game is something I'm very comfortable with. Like Might and Magic, much of the game takes place by exploring a large number of outdoor zones, rather than being held entirely within a single dungeon/tower.
The character development system allows players to either select a base class or create their own by selecting six major skills from a decent-sized pool. The other skills are still usable, but cost twice as much to advance. The skills are modified by typical attributes such as strength and intelligence. As characters gain levels, players are given five attribute points and ten skill points to allocate, and the skills that they used most frequently during the level also increase a little bit on their own. Skills are capped at twice their base attribute, conveying advantage to players who create focused characters, specializing on skills based around one or two attributes.
The alchemy and item enchanting systems are also pretty similar to what was seen in the Elder Scrolls games. Unlike the most recent Elder Scrolls game, however, your character's race and gender actually does impact dialog options and the available ways to resolve quests. There are even some choices and consequences in the game, where many quests have multiple conflicting resolutions and the completion of one quest makes the player fail competing ones.
It has a lot of area to explore for a portable game, especially if you're a completist. I'd guess most people would take around 40 hours on the game.
The setting is generic fantasy, but at least it's a more Sword and Sorcery flavored form of fantasy than the Tolkien-derived high fantasy games that are so common. There are no elves or halflings to play. The quests are generally more interesting and morally gray than is seen in a lot of other RPGs. There are quests involving infidelity, murder of relatives and lovers, political and religious corruption, incest and demonic compacts. The setting feels a bit gritty, more like those found in Fritz Leiber's "Fafhrd and the Gray Mouser" or R.E. Howard's "Conan" stories than those found in the politically correct and family-friendly high fantasy that is much more common now. Again, this reminded me a bit of the feeling that Daggerfall invoked, though The Quest wouldn't be considered as "mature" as The Witcher, as it is less over the top and there's no nudity or anything of the like.
I'm not overly happy with the control scheme and user interface. Movement is controlled either with an inconvenient swiping motion or awkwardly arranged buttons. This is especially true if you attempt to play the game in landscape mode. Many people have noted the poor controls in their reviews and on iPhone forums. The developers have commented that they plan to introduce new optional control schemes in the next release, so hopefully this will be addressed soon.
The game "plays nice" with other iPhone features, allowing you to listen to your iPod music or podcasts while playing and autosaving conveniently when you receive a call or text. You can immediately exit and return to the game at any time, even mid-combat, without having to take any additional steps such as saving or going through menus, making it convenient for the iPhone's highly distractible environment.
I wouldn't say it's my favorite game of all time, but it's certainly my favorite iPhone game at this time. It's definitely worth buying if you have an iPhone and have been toying with the idea of picking up an RPG for it.