Well, one side effect of Mart's Mutant Mod on Fallout 3 is that it aggravates an already aggravating condition... it's too easy to get to level 20. Just to give you some data points (hours played, level attained):

Vanilla Fallout 3

8 hrs, Lvl 5
20 hrs, Lvl 7
30 hrs, Lvl 10
40 hrs, Lvl 13
60 hrs, Lvl 20
80 hrs, Lvl 26
91 hrs, Lvl 30

Mart's Mutant Mod

7 hrs, Lvl 7
9 hrs, Lvl 8

At this rate, in another hour or two of gameplay and I'll be level 9/10, well ahead of the roughly 30 hours it took me without MMM. Or at least, it feels rushed and well ahead of normal speed due to all the additional encounters out in the wastelands.

My first play through took about 90 hours. That's not bad for getting to level 30, but I still feel like I only did about 1/4 to 1/3 of the entire wasteland / DC area, even though I hit about 3/4s of the quests. The problem is that once I hit 30, I lose a lot of interest in the game... so I need to slow down the rate of XP gain.

Looks like choose_XP (v2.4) is the mod that I'm looking for. From the description, it looks like you get an activator item upon leaving Vault 101, at which point you can choose the rate of XP gain in various increments above/below 100%. It's supposed to be compatible with the Broken Steel expansion, which a lot of other XP modification mods aren't. If I understand how it works, it doesn't mess with the amount of XP needed to level, but instead changes the amount of XP you gain when you complete a quest or kill something (similar to the +10% XP perk).

I think I'll try 33% of normal to start...

Updates:

- 33% feels slow at the start. I suggest leaving it at 100% of normal until you get to level 3. Then back down to 50% or 33%. I've spent about 8 hours, am almost level 5, and did all of Mothership Zeta along with a few odds and ends in Megaton. Next time, I'll leave the XP modifier at 100% until level 3 or 4.

- One side effect of 33% XP is that you are going to want to do a lot more exploration. In vanilla Fallout 3, exploration feels to be a bit of a waste because you'll get some much XP just from doing quests. So you tend to be quest-focused in regular Fallout 3.

- Overall, I think I'll be happy with either 33% or 50%. I could even see slowing it down a touch in the mid-teens.