I wouldn't mind them. It all depends on how fast I guess leveling is, as opposed to how fast a person can fall behind after a shift at work. They could be items that you find in game, or even postions that an alchemist could make; it all depends. They could be implements rather good or be abosolutely terribly implemented. It honestly doesn't bother me to have them in the game, same with the rested EXP. If someone wants to use them and get ahead, then let them. They will be the ones missing out on the adventure and the world around them; not to mention they more than likely won't find anyone that will group with them if they don't know how to play. It's honestly no skin off my nose if someone wants to jump ahead of me to get to an end game with no experience of the world around them. It won't be my fault when they can't get a group or when they don't know what spells do what.
There is also the fact that people pick up on things differently than others. Some people learn things faster than others and having someone sit at a level for the soul reason being to learn to play that character or that spell, then chances are they are going to get bored. Let's face it there are two types of bored people, people that quit and take their sub with them, or people that find ways to obther other players trying to play the game. In the end though, it still is there sub, so if they want to use it that way by drinking nothing but EXP potions or using EXP scrolls, then that's them; again, having no experience and only racing to the end to essentially say "first" will leave them with no friends and no experience to get into a group.
In one word, I guess it would be, sure. I really doens't bother me. If I use them or anyone else uses them, that's fine by me; if someone else doens't use them or I don't use them, that's fine by me too.
Situational
I think that they can be situationally good in some circumstances/ games depending on how other systems in the game function.
For example in modern MMOs where the trend is that all relevant content is experienced at max level, exp boosts are good. Especially when you have to climb 85+ levels to experience this content with other people.
However, if a game does a good job of making all content relevant even at the lower levels then I don't think there is a need for it. For example, Everquest has achievements and alternate advancements that keep previous content relevant. Some achievements provide lasting stats, buffs, AA points, or items. This system removes the need for faster leveling because lower-level content can also act as a way of facilitating max level character growth and progression.
* I think that for Pantheon it's going to depend on how substantial the death penalty becomes, and how relevant past and future content becomes over the proceeding expansions. If handled well exp boosts aren't something that should be needed/ wanted as a catch-up mechanic.
**Also, exp boosts should never be a pay to win type feature.