Alloy SHIVs aren't bad, but aren't particularly great. Having a portable source of cover with a mini-gun isn't bad, but you do give up a team member to get it
Hover SHIVs are, IMHO, worth having. They're the best scouts in the game (short of a well-played ghost assault) with 12 moves of flight by default, 24(!) if you get the double fly duration foundry upgrade (how many battles do you have that go past 24 rounds? And that assumes they move every round, staying still doesn't cost fuel.). They can fly around scouting the battlefield, are hard to hit and very durable, and can't be mind-controlled, which makes them ideal for spotting work.
Don't expect too much out of their weapons (even with the plasma upgrade), they're there to fly around spotting aliens so your real soldiers don't have to risk bad cover to do so. With the suppression upgrade and a good team of 5 backing one you can easily have an attack where the thing never actually shoots (just suppression fire), but contributes greatly to the team's success.
Also they pair very well with Archangel Squadsight snipers on outdoor maps .