Eh. It depends on your players, honestly. I played mechanic-less PbP for a long time before I got into anything with mechanics. And there's a wide range of what, exactly, that means. On the one hand you have the journal RPs, which I'd say are farther away from what PbP is from here, and then you have single-threaded forum RPs, which is basically exactly what we do here, but without as much mechanics focus, and on the third hand, you have multi-threaded forum RPs, which is basically what we have here, but each line of story is a separate thread.
From my own experience, which is limited, of course, single-threaded forum RPs can have problems keeping things clear as to what's happening where and with who. Multi-threaded forum RPs can have the problem of looking like there's not a lot going on so people don't post because not a lot's going on. Both of them can have problems with players who always think they should succeed, which is my main reason on souring on the whole mechanic-less route. This is probably more of a problem of poor admins than it is a problem of the type of game, but I prefer mechanics now.
Also, I'd say, it depends on what you're looking for in a PbP roleplay whether a mechanic-less one will suit you. Most (but I'm definitely not saying all) of the multi-threaded ones I ran into have a lot of just sitting around doing character interaction. Plot and a gripping story was not really the focus, except on rare occasions. Single-threaded PbP tended more towards the straight plot, but, without a core admin or proactive and creative players, it would devolve into traction as reactive people didn't take the story anywhere new.
As an aside, I've only actually RPed once, face-to-face, and that was in the previous year, but I've been doing tabletop online through IRC for about five or six years. Mechanic-less PbP for about five years before that. Much of my playing experience is with the fan-created Final Fantasy RPG, but I've also played a little Shadowrun and even less Fantasy Craft. I've GMed the Serenity RPG and a Cortex fantasy homebrew of my own making. Even in mechanic-less RP though, I tend towards the GM role.