
Shall we get started?
Pros
Ding! Ding! Dingdingdingding...
You gain triple experience from both mobs and quests. This causes levels to speed by, and you can have a level 60 in a couple days with little effort (level 60 is when triple experience ends). A red quest for SM can give around 17550 experience, catapulting a level 31 to level 32. Stocks runs can give two or three levels, both from mob experience and quest turn-ins, even when the run is lead by a level 70.
LFM...NOT!
The great advantage of the triple experience requires that you be grouped with the linked account and be within a certain level range. This encourages group play. Besides enabling the buddy system on a PvP server (where you're less likely to be ganked unless they're ridiculously higher level than you), it also means less downtime in the form of looking for members for a group quest or dangers of high-level mobs catching you unawares. If you pick classes that compliment each other, you gain a lot of experience and can adapt to many different situations. Also, you can get higher level quests done at a lower level, increasing the xp gain you receive from turning them in.
New Class, No Hassle
If you've always wanted to play X class but never had time to seriously level an alt, the triple experience smooths the way for you. Even if you simply need an alt at high level so you can get them max rank of a beneficial profession, such as alchemy or inscription, that you just don't have room for on your main, this is an effective means of gaining it. As well, you can try out a role you've never filled before, such as tanking instead of healing or DPS.
Cons
Penniless and Friendless
If your friend is using an account, likely they have no money. Triple experience gains speed you through questing areas and levels like no other, but what comes with levels? Skills and spells that need to be trained. Training costs progressively more and more money, but you're not doing enough quests to make that money. Therefore, it's an intense monetary burden upon the person who recruited the friend to supply money for both rapidly-levelling characters so that they can train and improve. In addition, since less quests are needed for levelling, reputation gains falter behind. If you want a certain class and race but want another race's mount, this is a detriment.
That Look is SO Last Month
You're doing less quests, making less money, and blowing through zones. At some point, you're going to notice your greens from VC and WC just aren't cutting it, and you haven't been picking much up along the way. You're stuck; do you keep going as you have, do you rope a 70 into some instance runs, do you grab a group and hope what you want drops and no one else needs it, or do you buy yourself some equivalent level greens which you also may/may not outstrip in just a few hours? It's a difficult question.
What Does This Button Do?
When in a group, mobs often die fairly quickly, before you're even through a full rotation. Group settings against squishy mobs often favor burst damage moves rather than a damage-over-time or a debuff setup. You can settle into a pattern of what works and works quickly, especially if you notice your partner is killing the mob before your DoT gets two ticks. You're training skills, but you're never using them. Some don't get bound or even read. I cannot speak personally for everyone who uses this levelling system, but my lock is at level 32/33 at the moment, and I only have a basic grasp of how to play her. I do not know what all my buttons are for or how to use them in a real rotation. I barely even know how to spec, because if I take too long to make a decision, the points pile up. Though my partner does not have the same difficulty (he's played a paladin before), I'm looking at all the different curses and have only the barest grasp of what works in what situations.
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There are pros and cons to every system, as is evident. Of course, there are the bonuses of the free month and the Zhevra Mount, and it's all going to amount to personal choice. I'm going to feel incredibly noob on my lock and every other character levelled this way for a while, but hopefully I can figure it out.
All comments and additions to the list welcome.
Also, opinions: are there any races that actually look good on the Zhevra?