If you mean "boom" as in make the award on the spot - you have some issues when it comes to leveling up -
Getting new spells in your spellbook just seems problematic when it would have to happen at near instantaneous speeds.
I see little to no problem "updating your character at the table."
Most of the time - this takes less than 30 minutes or so - unless you are having to make some serious choices - such as the possibility of multiclassing into either a PrC or a normal class.
If I am "updating" a non-arcane caster, I can usually do this within 5 minutes. PrC's - depending on what is available; complicate things.
Now - if your GM is considering just "awarding XP" without regard to doing the mathematics involved; this is generally a BAD idea.
I have seen one group imparicular where this leads to a whole lot of issues. One thing is for sure - NEVER make XP awards a "percentage of going up a level", as the lower level members get the serious shaft in this setup. I saw one league where this happened - and that absolutely stunk on ice (expletives come to mind - but you get the point)
Granted - you can do some spontaneous awards for stories, goals or "non combat" type encounters; but it is usually better to have a "mathematically objective" method - than an ad hoc one. A GM is advised to be careful with this however, because some encounters (such as one that involves securing aid, helpful information or other assistance) can be "rewards" in and of themselves.
My "test" is as follows
The following questions must be answered in the negative:
1. Does achieving this goal involve combat of some kind. If so, award the combat XP
2. Does achieving this goal involve a gain in and of itself - such as securing valulable information, material, or assistance (e.g. free spells cast on your behalf, allies to assist in "jobs" (thieves guilds ect.)? If so - the gain itself is reward sufficent for the PC to be doing it.
The following questions must be answered in the affirmative:
3. Does achieving this goal involve some risk to the PC's person or possessions other than combat? (ex - a spying mission of some kind) If so, this is a candidate.
4. Is this sort of goal unachievable in merely one adventure? If so - a story award may be appropriate.