This perk "advances the character to the next experience level, if available, with all advantages that entails" - Except one advantage. The level you just gained has no perk, as Here and Now effectively disappears after giving you another level. In simpler terms, you level up again at the cost of a perk.
In Fallout 3, you cannot take this perk at level 20, unless you have Broken Steel, in which case you cannot take this perk at level 30.
In Fallout: New Vegas, if you do the perk glitch of obtaining more than 15 perks, you can use the Here and Now perk to stop from leveling up from level 10 onward. You still lose out on a more beneficial perk, but now you can get experience points like a normal game after level 10.
This perk will not work past level 34, because each DLC increases the level cap by 5. Since the toolset cannot load all installed DLC at once, it is difficult to determine the actual level allowed. You can still calculate and set it manually if you have access to the toolset (calculate number of DLCs installed * 5)
The amount of XP given to you by this perk is affected by the Skilled trait, meaning it will not increase your level (usually leaving you 10-20XP short).
In Version 1.0 of Fallout 2, this perk gives you an amount of experience equal to the amount you have at the level you take the perk, rather than leveling you up one level.
If the player wishes to pursue a neutral karma path to obtain the achievements at the respected levels, Here And Now can help jump to the next level in case the player wishes not to risk waiting another level where they might have to make karma decisions that dip down to evil or above to good karma in addition to gaining XP, which might lead to accidentally getting to the desired level with evil or good karma rather than neutral.