Something worth noting with the more mana expensive cards, is that if they do nothing the turn they are played they are extremely vulnerable to removal. Being vulnerable to removal and costing a lot of mana is hurt. Opening Gambits/End of Turn Effects circumvent this by instantly generating some value for you before your opponent can destroy it. Think Dancing Blades. Most of the time you need to use removal on it, but if it already killed something with OGambit then it trades 2 for 1.
If you drop down a Grailmaster behind you and your opponent can’t remove it then it is going to wreak havoc. Summoning 3 minions during your next turn is easily doable and you are almost guaranteed to get Ranged, or some other extremely potent combo of keywords(Flying/Celerity, Flying/Frenzy, etc…) you are pretty much guaranteed to win if not then it will be a huge lead generated.
However if they destroy it with a spell then you have nothing. Just about all removals cost less than 7 Mana so you are behind a few mana on tempo.
Now take a look at Pandora. You get -3 ATK and +4 HP. 10HP is a ton. You pretty much can not kill it by trading into it, it demands spell removal. At the end of your turn you also get a 3/3 Wolf.(4 out of 5 abilities are useful in just about any situation.) If they remove pandora(roughly 4 mana or so), then you still have a Wolf that they need to deal with. I would guess that a Wolf is roughly 4 mana worth of tempo, and they will often need to commit another card to deal with it.
You will be even or ahead on tempo if they remove Pandora with a spell, and it will often trade card advantage 2 for 1 in your favor. Yes Pandora is kinda slow, but I think it is a strong card. Against aggro it can still be useful as you can use it to body block. If silenced a 3/10 body is still a very strong force, especially if you have any sort of buffs.