If you don't have a problem with touch controls, Doom Classic is an excellent port.
Firstly, it must be noted that this is actually a port of The Ultimate Doom. This is a good thing as this was id Software's 1995 release that included the entire original Doom plus a brand new chapter titled Thy Flesh Consumed, so you're getting more bang for your buck, and it seems to be a direct port of the PC game. It's pleasing to say that everything is in tact. No sacrifices have been made; you can expect to play the entire original game in all it's glory. Nothing has been cut out or simplified like you saw in many of Doom's original console ports. Every level is there and every enemy remains. In this day and age you simply don't need to worry about console ports of old classics, even if the platform in question is your phone!
The most important question of course, is how well do the controls work? Well, much better than I expected. I was skeptical at first having played Duke Nukem 3D on the iPhone, which, in my opinion, was very awkward and sensitive controls-wise, and there was certainly room for improvement (which could have been possible). Doom on the other hand has plenty of customizable controls with some great default setups. I personally think that the directional pad and wheel setup works best as it's actually possible to navigate the levels with ease. In fact, the only issue I have with the controls is the fact that you can't circle-strafe whilst shooting as you have to take your thumb off the turning wheel to shoot. However, it isn't possible to circle-strafe and shoot with touch controls anyhow, and you can still strafe and shoot normally with no bother at all. So really, these are the best the controls can be considering that it uses a touch screen. You just need to keep in mind that there will always be limitations regarding touch controls in first person shooters, and this will no doubt put some people off regardless.
Doom Classic is flawless visually along with the rest of it's performance. The game uses OpenGL rendering along with the Xbox Live Arcade version, so the textures are crisp and clear with little pixelation unlike every other Doom port, and the resolution is much higher. If you compare this port to the original PC DOS game, it actually looks a heck of a lot better than the former. I did notice one or two temporary graphical bugs every-so-often, but it's no big deal. The frame-rate is smooth and consistent throughout, and I didn't encounter any issues regarding it whatsoever. The audio is just about perfect too – everything has survived the translation in the music and sound department, although I did notice that the odd track does sound slightly different to it's original counterpart, though this isn't really a complaint.
The only improvement I would suggest is a port of Doom's multiplayer deathmatch and co-op mode via the internet, but I guess that's just me asking too much. I heard that initially the earlier versions of the port DID have this function, but I never played the earlier versions, as it's only been recently I even got my iPhone. This may put some players off, but honestly, if you're a gamer and like me, you love Doom and your FPS's in general, it shouldn't bother you.
Other than that however, Doom Classic is an excellent iPhone/touch port. I've stated that as of Version 2.1, everything is in place, the controls work just fine as far touch controls go and it's top-notch on the visual front. Although the game is more expensive than your average iPhone app (I paid 2.99 in the UK pound, and your average app costs 0.69p), this is still a perfectly reasonable price considering Doom is a classic game, and this port is complete in just about every aspect. If you're a Doom fan and you've got an iPhone, there isn't really a better choice if you want to play Doom on the go. Here's hoping Doom II will arrive on the iPhone soon!
Wrap-up:
+ Complete version of the Ultimate Doom on your phone
+ Identical to the PC original in terms of content
+ Nice high-resolution, smooth gameplay
+ Touch controls can be customized and aren't as dire as you might expect...
- ...but they will take some getting used to regardless of how well you play games on a touch screen
- No kind of multiplayer option available
- Only tested it on the iPhone 4, so I'm unaware of how well it performs on older models