The other day, Beedo wrote a defense of the halfling on mechanical grounds, arguing that with its superior skills, HP, and saves, it filled a vital role as a durable scout that a standard thief does not have the survivability for. After some consideration, I realized that the Dwarven Delver from the Player's Companion essentially fills the same role - higher skills, higher saves, and more HP than a normal thief, at the cost of leveling rate and variety of skills.
After actually looking at the Delver class for a good while, though, I realized that it is almost everything I want in a stealthy class. It's durable enough that I think it might actually survive, but it also levels at the same rate as a fighter. Varied leveling rates are all well and good, but as others have noted before, having the thief be "a crappy class whose main strength is rapid leveling" is not particularly good design. The delver's skill selection covers the main bases of thieving, and there is one omission I especially like. The delver can find traps on a 14+ at 1st level, and gets better at it over time... but he does not have access to Remove Traps as a skill. This meshes beautifully with the way I like to run traps; realizing that there is a trap on something is a game of intuition and clues, and I'm fine with having skill rolls to realize that there's definitely a trap. I prefer to run the disarming or circumvention of traps in a more creative fashion, sans skill rolls. If your thieves have Remove Traps as a skill, they get unhappy when you don't really want to let them disarm traps via die roll. But, if you have a delver instead of a thief, there is no problem.
I have a few very minor nits to pick with the class, but they're not all that bad. Lack of Open Lock is pretty annoying; I get that it fits with the class' flavor, but Open Lock is a neat mechanic because when you find a locked chest in the dungeon, you have a quiet way to try to get it open. Lacking Open Lock means that you're going to have to resort either to bashing, which is loud and brings down the wandering monsters, or to carrying the chest out of the dungeon, which may not be feasible due to encumbrance constraints. I therefore feel like Open Lock is one of the thief's essential functions and, left to my own devices, would probably replace Climb Walls with it on the Delver, mess with the flavor text a little, and add the Climbing proficiency to their class list (maybe in place of Mapping).
The other place where the Delver has a potential problem compared to the thief is in weapon selection. Missiles, plus hammers, axes, picks, and flails. Notably absent from this list is that prince of weapons, the spear, which normal thieves receive access to. Part of the problem with this is that I'm fairly sure that at low levels, the best way for a thief to survive to 2nd or 3rd level is with a spear in the second row, while the fighter and cleric take point until traps are suspected. If you can pick up a fighter henchmen with Combat Trickery (Knock down), you can sit in the second row and backstab prone opponents from behind the shield wall, too.
The delver, however, lacks the access to reach weapons required to pull this off safely from the second row, so he's stuck either in the front row with low AC, or in the rear with a missile weapon and Precise Shooting's penalty to hit. He does get Combat Trickery (Knockdown), so he could try to generate his own backstabs, but a design solution (again drifting from the original concept of the class) would be to drop Caving and shift that 'custom power' back into broad weapon proficiencies, which is where I'm pretty sure it came from during class design.
In any case, though, I'm still convinced that the Delver is fairly close to what I'd want in a thief class, and that with a few tweaks to 'make it my own', it would be something that both I and my players would seriously consider during class selection, rather than relegating it to "NPC status" like the core thief.