Huge performance change in terms of load times. Opening/navigating inventory, triumphs, bounties, etc smooth and quick. No more floating in the air between loading zones. Less than 30 seconds to load into tower or other destinations (down from 1-1.5mins).
Clearing xbox cache was an improvement but nothing like moving D2 over to a SSD.
Picked this up at BestBuy this last weekend if you are looking for a suggestion: https://www.bestbuy.com/site/pny-elite-240gb-external-usb-3-0-portable-solid-state-drive-aluminium/5594802.p?skuId=5594802
(Also, I am in no way affiliated to PNY or BestBuy)
Hope that helps confirm anyone thinking about trying this!