Millenium DI-E

I bought one of these to test, because I couldn't believe how cheap they were.
Plugged something in, and it sounds fine. These aren't esoteric devices that add "magic" to anything that passes through. They make an unbalanced 1/4" jack into a balanced XLR.
Despite being transformer-coupled, the low-frequency transmission is excellent. My mixing desk (capable of +22dBU output) didn't cause any saturation distortion in the transformer - I had to find a fairly large power amp and apply 20Hz sine waves before any distortion showed up.
I was very impressed with the tests, so I bought lots more and keep them for festival stages. They work every time (no active electronic circuitry to go wrong) and never degrade the sound.
That's all I can ever ask of a DI box.

In the studio or on stage, this is a great sidekick for almost any musician, engineer or amateur. It's simple and very durable (truly "built like a tank" as the saying goes). With regards to features, as described it includes one jack input, another jack output (a link, as it were) and an XLR output, as well as a ground/lift switch and an inst/speaker switch for the link output. Being a passive DI box, it's of course useful for active instruments, but it does a surprisingly good job with passive ones as well from what I can tell. You can get really creative with all sorts of setups with a few of these, so I definitely recommend giving them out a try - be it for interesting signal chains or just general noise cancellation, they're astounding for the price.

I use these all the time in my job as a university theatre technician and in my freelance life as a gigging musician and sound engineer.
The main attraction, it has to be said, is the price. I can get three for the price of the nearest competitor, and the quality is pretty consistent with those competitors. I have had one problem with the old version of this product - the xlr out came loose from the box. However sound wise it still worked and I was able to patch it up and keep using it. Since then, they have changed the design of these and I have not had any problems since. Never really worked out what the instr/spkr button does. Wouldn't fancy trying a speaker load on this - surely this could fry your valve amp? I guess you could use the link out into your cab, but don't take my word for it! I tend to use it as a mute switch, which in itself is quite useful.
I like the fact that these are small too - I keep one in the car and one in my guitar case always. Even if you prefer expensive DIs - you can't lose having one of these as a backup.

It's super hard to fault this product. I've been using it for the last year-or-so to record my guitar and bass, and it has worked fine with no issues. The DI box is basic, with few features, but splits the guitar signal with absolutely no problem at all.
Pros:
+Easy to use, as it requires no external power and has very few controls. This makes it perfect for beginners
+Very cheap, and does the job perfectly - the product is very good value
+Includes a ground lift swift to cut out any background humming when recording.
+Sound quality is flawless, and has no noticeable decrease in quality.
+The build quality is strong and has survived well for the time I've used it.
Cons:
-There are few advanced features - this wasn't an issue for me but could be for more advanced users
-Could benefit from a little instruction manual to help new users who've never used a DI box. Setting up the box was slightly confusing
I strongly suggest that any young producers or guitarists wanting to get into recording demos or full tracks get this product - it has helped my music playing and production skills develop in many ways.