I'm currently DMing a group of six 15th level characters (having worked their way up from level 1 starting in 2001). One of the somewhat newer characters (started last summer as a replacement character, has around 35 sessions under his belt) is a Monk / Forsaker and refuses ALL magic (i.e. attempts saving throws against Haste spells, even a Heal spell used against his will to save his life). He does not use the Vow of Poverty and we're a very "by the book" standard group (no house rules, although I allowed the character to switch between Monk / Forsaker without penalty). I don't do anything from a DM standpoint to take his lack of magic use into account.
The character is very effective. He's not a heavy damage hitter (leave that to the fighter), but his grappling ability as well as his ability to disarm opponents is *very* effective. Any caster on the ground and without Freedom of Movement is in danger. Anyone with an uber weapon can find it in the monk's hands if they roll poorly (I routinely stat out the bad guy's secondary weapons these days and make sure I put their grapple score on my DM initiative cards). With the character's damage reduction & spell resistance, great hit points (just shy of 200), mobility/movement, he holds his own pretty well against those decked out with items and is one of the more creative fighter types I've seen played.
Downsides are that he heals slower than other characters, although this liability is not too bad through a feat from The Complete Warrior. Against creatures with natural weapons and that there is no reason to grapple (i.e. undead), he is more like a secondary fighter. Otherwise, he's as useful as the primary fighter in combat and the player really enjoys the character.
Just my 2 cents...