I bought the Zoom H4n a little over eight months ago and have been using it constantly since then, so I thought it was time to write a proper review. I did plenty of research before taking the plunge on this one, looking at some of the Sony models, Marantz etc. before settling for the Zoom. Why? Two things: sound quality, and functionality.
I'll address those two items shortly, but first of all, what do you get when you purchase the H4n? Well, obviously you get the recorder itself, and also in the box is a rigid plastic carrying case to protect it, a 1GB SD memory card to get you up and running, a mic stand adapter, a USB cable to connect it to a PC, an AC adapter so you can run it off the mains, and a windscreen. The recorder itself feels solid and well-built, with a tactile rubberised finish that makes it sit comfortably in the hand. There's a tripod mount at the back so you can rig it up in a live environment, and you can purchase a little remote control that lets you operate everything without touching the Zoom, which can be handy if you're using the mics and don't want to pick up the sound of button presses etc. Oh yes, there's a comprehensive manual in the box, which is much better than a CD with a PDF on it, let's be honest.
Now, sound quality - it is, in a word, stunning. I've been using the H4n mainly for field recordings (ambient sounds, the rhythm and hum of cities, etc.) and the recording quality is absolutely amazing, with every nuance subtly picked out and placed in the stereo field. Recording levels are of course adjustable to suit the dynamics of each situation, so you can very easily drop the levels down to avoid clipping, and there are plenty of recording modes as well, allowing you to record in WAV or MP3 formats, at various quality levels (and corresponding sound file sizes). Unless you're just recording a quick podcast that you want to dump straight onto the web, you'll probably want to use the WAV format for the highest quality. The default WAV format is set 44.1kHz/16bit (CD quality), which is a great starting point, and you can go as high as 96kHz/24bit, which effectively gives you 100x the resolution of standard CD. Plug a pair of decent headphones into this baby and listen to some ambient recordings - it is stunning, and puts you right in the heart of the hi fidelity sound field.
I mentioned I do a lot of field recordings; the H4n has a built-in pair of very, very high quality microphones in X-Y format, crossed and positioned to completely eliminate (to all intents and purposes) any phasing. You can switch them from the default 90° field to a wider 120° field if you need to capture sounds across a wider area; just turn the mic heads round until they click into place. One thing worth noting is that the unit as it stands is very susceptible to picking up wind noise - the provided windscreen helps to a certain extent, but it still lets through an awful lot of noise, so you'll want to invest in a Rycote or Redhead wind jacket for it if you plan on doing any recording outside. They aren't cheap, but you can buy them specifically for the Zoom H4n and they work perfectly.
As far as functionality goes, the Zoom just cannot be matched by anything in its price range, nor (as far as I've found) anything costing far more. It features a pair of inputs for external devices, and I've plugged my electric guitar straight in, used a Shure SM58 mic, keyboards etc., and they are all "plug and play" and sound terrific. The H4n has a built in multitrack recorder with four tracks in total, and you can bounce from one track to another, overdub etc. with minimal effort, and of course no loss in sound quality like the old days of tape. You can record on all four tracks simultaneously and do all your mixing from within the H4n. For getting down ideas, riffs, or even complete songs, it's a treat. Oh yes, you can also plug the H4n into your PC via USB and set it to either act as a disk device, so that you can just drag/drop your sound files onto your PC for editing, or - get this - use it as an ultra high quality sound card! There are drivers for Windows 7 on the Zoom website.
Pair the H4n with an HD camcorder or a digital SLR with video and you've got yourself a broadcast quality setup.
The only downside I can think of is that the battery life is a bit slim, at about 5-6 hours with 2000mAh rechargeables, or about 10 hours if you engage the "stamina mode" by flipping a switch inside the battery compartment; that shuts down some functions and limits you to CD quality WAV files as the highest quality recording format available. This is significantly less than some of the Sony models, but it's no big deal unless you're going to be doing a lot of recording in one go and don't have access to a recharger.
I'm super pleased with my purchase, and I'm looking forward to many years of use out of this little beauty. If you're travelling to the US by the way, you can get the H4n for the same money in dollars as you pay in pounds over here, so you save about a third off the UK price, worth thinking about if you're on a business trip or holiday. Finally, I can highly recommend seeking out a training DVD for the H4n, which is produced by a guy called David Wills (a pro audio dude), and he takes you through everything it does, step by step, over about two hours - definitely worth investing in to get you up and running and help you get the best out of it.