You can pretty much unlock any GSM device for use on other carriers. Although you will be missing some carrier exclusive offerings (4glte or Ltz for example) and will be relegated to 3g. Not such a bad deal.
But the root record has to go to Acer Iconia A550/A501. (ok, it's not a phone). Copy small apk to sd, install, reboot. Rooted!!!! I timed my self at less than 90 seconds! My HTC Desire took 5 minutes.
No Kyes, no Odin, no typing commands and no ADB. That's what makes for an easy root.
Yes it is true you can unlock any gsm device, however, some phones need an unlock code from either the carrier *free*, or an unlocking service. As for rooting the sgs2 yes you need Odin just to flash a kernel, but for the unlock, no commands, no adb, just a simple tool by Helroz called the GS Unlock without needing a pc, odin or adb to do for unlocking. Yes you will lose some of the carrier exclusive offerings, even using att as my current carrier, I do not use their apps at all pre-loaded on my phone. However when att launches their lte network, I will have to get a new phone if I want the higher speeds that lte provides, otherwise I am stuck on the hspa+ which honestly is not that bad at all. Verizon Wireless has LTE, but I am not sure if their lte phones will work on other providers just yet as from what I have heard has both the cdma and lte radios. As for the easiest rooting experience, Samsung GS Captivate *just download a flash when the phone was running Eclair*, rename the file as update.zip, reboot into recovery, flash update.zip reboot the phone rooted in less then 2 minuets, the sgs2 is a bit different, however, just needs odin to do it to get CWM and Rooting to work properly. The only reason to Kies is just for drivers for the sgs2 otherwise it is a useless program in my opinion.
Just my point is not all gsm phones are exactly 100% free for unlocking especially if you are not a customer of the carrier the phone is branded, then you have to actually go through an unlocking service unless the phone has the unlock code imbedded into it's firmware like Samsung does.
HTH,
Charlie