A lot of people think that using Adrenaline on anything over a 3-power creature is dumb, but there are a few other aspects of Adrenaline to think about.
First of all, Adrenaline is all about adding damage/turn, just like playing a creature or a poison card. In terms of raw damage/turn, there are two separate points at which Adrenaline will add it's maximum of 9 damage/turn: at 3 Power, when it adds 3x3 damage, and at 8 power, where it adds 6 and 3 damage, for the same total of 9. 7 Power is second-best at 8 extra damage/turn, which is why Phase Recluses seem popular with Adrenaline as well: there just aren't any 8-power creatures that are nearly as cheap as the Phase Recluse.
Secondly, of course, there are the special-ability uses of Adrenaline. Mana-creating creatures, Flying Druid Staves, Vampires, and with lesser effect Devourers, Puffer Fish and Flying Arsenics all produce increased effects under Adrenaline. These uses alone can make Adrenaline a useful addition to a deck if they're properly planned for.
Finally, there are the environmental interactions that you can use to your benefit if you're careful. The obvious are it's uses in penetrating Congeal/Frost Shield and Procrastination, but you can also use Adrenaline offensively if you use Toadfish/Parasite, Plagues, and Aflatoxin (both to hurry up the poisoning and a 2nd Adrenaline to make their field fill two turns faster by making the Malignant Cells expand faster.)
All told, Adrenaline is a card that requires forethought and wisdom to use correctly, but can be a real game-swinger if you know what you're doing with it.
Sample Fun Deck, all Unupgraded but with 5 rares:
Mark of Life
4 Emerald Pillar
10 Wind Pillar
4 Firefly Queen
2 Owl's Eye
2 Flying Weapon
3 Adrenaline
2 Druid Staff
2 Empathic Bond
1 Miracle
Use the Adrenaline on a Flying Staff for massive lifegain, or on a normal Firefly for 12 damage and 4 Light Mana per turn - it's easy to power that Miracle this way - or on a FFQ to turn her into a 12dmg/turn badass attacker in addition to her decent toughness and summoning skill. Owl's Eye adds control in a pinch, and between that and all of the massive lifegain, you're likely to lose to deckout rather than HP loss with this Adrenaline-powered beast.
