1. The unlocking DOES work with the newest firmware. Be very careful about entering your numbers: Don't mistake "5" for "S" or "0" for "O" for example. The best/easiest way to get the key into the scope is to use something like telnet over a LAN connection to the scope. This way you can store your valid unlocking key, and the SCPI commands to install and uninstall, in a text file, from which you can then simply copy-paste the correct code and the SCPI commands into the telnet application.
:SYSTem:OPTion:INSTall XXXXXXXXXXXXXXXXXXXXXXXXXXXX
to install the 28-character key (4 groups of 7 chars but with no hyphens)
:SYSTem:OPTion:UNINSTall
to go back to stock 1054z with no options (or "trial options" if you haven't run out your trial period yet)
If you make three mistakes in a row trying to install the code to your scope, you will be "locked out" and you must leave the scope turned on for 12 hours until the "penalty time" runs out before trying again.
If the serial number of the scope is hard to read on the "System Info" screen, look at the sticker on the back of the scope, or in your original calibration document that came with the scope. Carefully copy the SN into a text file and save it, so that you can just copy-paste it when you need to use it in the future (like entering it into the keygen program.)
2. The unlocking code DSER (all options except 500uV vertical) is not documented or listed in the keygen program for the DS1054Z, but it works anyway. Don't even try the 500uV/div Vertical feature, the hardware of the 1054z does not support it and it can cause problems. I suggest downloading the .zip file from the website and using the stand-alone applications that are in the zipfile. This way you will not be dependent on the website, you can run the keygen locally on your own computer. The zipfile includes a working Windows stand-alone executable, an HTML file version that will open in your browser, a Linux version that can be compiled and run on your linux distro, and an OSX version (I have not tested the OSX but I can confirm that the other three work properly.)
3. The horizontal resolution does not change; the fastest timebase setting available will still be 5 ns/div after unlocking. The scope will report that it is now a DS1104Z when asked.
4. The higher bandwidth means that you will get more accurate display of high-frequency signals. The right way to test this is to use properly terminated, proper impedance cable connections from your signal source to the scope (not the "150MHz" probes that come with the scope) and a signal generator that can actually produce a fast risetime square wave or a sine wave of known amplitude. Since the DS1054Z has only a fixed 1megohm input impedance, you may need a 50 ohm feedthrough terminator-adapter or a 50 ohm T plus a 50 ohm terminator at the scope end of the 50 ohm connecting BNC cable from your 50 ohm output impedance signal generator. Some people have tested the bandwidth (looking for the -3dB rolloff frequency of a known amplitude sine wave input) and have found that the scope actually exceeds the 50 MHz spec before, and exceeds the 100 MHz spec after, the unlocking.
Be sure to let the scope warm up completely and run the Self Calibration routine after applying the unlock key.