I also buy key ring bundles once in a while, and use those ultimate upgrades exclusively on those items.
I.e. my Iconian Space Set was upgraded this way.
Personally I don't think the other items are worth using ultimates.
On the other hand I really enjoyed the Phoenix Upgrades from the last 2 Phoenix Box event.
Basically I try to save as much Dil as possible for the next event, dump it all on Boxes, and turn
everything into upgrades, minus whatever I want/need. Then I wait for the next upgrade Weekend
to use all of those. This has served me well so far, and I also got lucky once and nabbed an Epic token.
I am hoping that we get another event in June or July.