Reliably converting the 3.3V levels of the Raspi GPIO's to 5V and vice-versa on ALL 17 available GPIO's.
(well, all 17 GPIO's on P1, the 4 additional GPIO's on P5 are not available to the ConvertPi).
Protects the 3.3V only GPIO's of the raspi when working with 5V devices.

Works as advertised and fits (mechanically) well onto the raspi.

Leds are a little dim, but good to see. (On the other hand, you don't want to waste 20mA on each of them.)
Actually I love the Leds as it is quite easy to see, if there is something going on (it it is slow enough for the eye to catch).
Also the arrangement of the leds as the same as the corresponding GPIO in the header and the silkscreen is naming all of them in a quite readable way. This is a really handy feature, as I can't remember the pinout of the raspi-GPIO's.
Now I just need to look at my ConvertPi. ;)

Also, all convert GPIO's together with GND, 3.3V and 5V are brought o the small prototyping area.
Unfortunately there was not enough space to label them there as well, but so far I didn't need that.

One thing to note:
The level translation works in both ways (bidirectional), but all GPIO's configured for input will be (weakly) pulled high!
I didn't expect that, but it does not influence the operation of the ConvertPi.
You just need to know that any GPIO which is not driven low acts as if it has weak pullups.

I tested it with the I2C (100KHz), SPI (up to 10MHz) and the generic GPIO's (several kHz) for several days.
(with both I2C and SPI /CS the pullup-behaviour is quite a bonus as the idle state of these signals is high....)
So far I didn't encounter any problems and I don't expect any to appear.

The only thing to note is the SLOW delivery time (around 6 weeks).
But this is not a property of the ConvertPi, but of the shipping used.
If you need one ConvertPi urgently, I'm sure, Matt will come up with something faster (and more expensive).