Since everyone is concerned about it, I might as well pitch in favor of such a talent
More seriously, one factor to weight in is how you hand out experience point in your campaign. If you hand experience by the rules (skill xp, spell xp, kill xp, hit xp, crit xp + ideas), I would consider such a talent far less powerful than if you assing a flat number of xp which is equal for all players per session. Why? Being higher levels means diminishing xp gain from kills and spells. The diminishing return on xp gain from being higher level will however have no part in a flat-xp hand out campaign, so you may want to take that into account.
Incidentally, buying other talents has an impact on xp gains, indirectly speaking. Some guy who gets +30 BO will obviously get more kill and hit xp, while a caster might manage to gain more xp simply by virtue of having merits giving spell bonuses (allowing him to caster high level spells easier) or giving more power points. So having a talent for experience gain isn't that unbalancing so long as the extra xp gain isn't overwhelming.
Lets say for the sake of argument we go with a Minor talent that gives 25% extra experience, and a major talent that gives 50%.
Just an exemple of what it actually means if all xp is multiplied, even the level 1 starting xp:
Level 5: Minor (not quite 6th level), Major (halfway between 6th and 7th level)
Level 10: Minor (level 11), Major (halfway between level 12 and 13)
Level 15: Minor (close to 17th), Major (3/4th to 19th)
Level 20: Minor (halfway between 22nd and 23rd), Major (25th level).
This is in a flat-xp campaign, and as I said, the difference should actually be lower than that in a campaign where standard xp gain is allowed for reasons mentionned above.
It is fairly non powerful at low level, but the difference in level gets more important as one ranks up. But then again, this comes at the cost of not having another talent. Do you allow talents to be bought with DPs in your campaign? If you do, what is the cost? If for exemple, you go for a 1 DP: 1 TP ratio, then he might easily buy back what he spent with his extra levels and still have point to spend and then the talent might be unbalanced. Of course, I'd also argue that a 1 DP = 1 TP ratio is unbalanced in itself =) If you go with an experience cost rather than a DP cost, then the question is at least in part, how long will your campaign last (and therefore how much extra xp could he spend on talents and remain at the same level as the rest of the party)?
If you don't allow it at all, its unlikely the character will be that much more powerful than those of the other player. The difference between a level 10 and a level 11 when the level 10 has an extra minor talent to play with seems pretty fair to me. Even as a caster, at high level, I might prefer to be lvl 20 with Aura(minor) than level 22 without it. The level 25th will have an edge because he unlocks a fairly powerful spell, but when it comes to arms user, 25th level fighter vs 20th level fighter with a +30 BO talent to compensate, I'm not sure - everything else being equal - the higher level would win. I would avoid, of course, going with a +100% xp gain bonus. That might be too much of an edge whether you're playing low or high level.
