Normally, these boots just increase your walking rate (that is, you run), but I discovered that at higher levels, they give your warriors an extra half-attack. Am I crazy?

You're correct. Boots of speed actually permanently put the character under a haste spell.

dispel magic wont have an effect but the mage spell slow will

Try casting free action on yourself, then equipping the boots of speed (or play a monk). You'll be immune to the boots.

If you are slowed while wearing the boots, the effects cancel each other. If you re-wear the boots after this, the slow spell effect is lost, leaving you hasted again.
It's what I've noticed, though.

The last game I played, two of my characters were wearing the boots of speed and I found it really annoying that those two characters got everywhere before the rest of my party. This happened sometimes where my two characters would be isolated in a battle and the rest of the party just strolling behind while my characters were getting massacred. So I just usually sold them. The extra half attack isn't really worth it unless your a high level fighter/barbarian and I always found myself dualing out of boredom.

I am quite certain that boots of speed do not give any extra attacks, just a faster movement rate. At level 13+ (I think) fighter types get an extra half attack per round. Perhaps you're confusing this with what you think boots of speed may have given you.

IIRC, there is a bug with the Boots of Speed. If you have, say, 2½ attacks, the boots will round it up to the next integer amaount, in this case, 3. If if you start with an even integer # of attacks, the Boots don't do anything.

There are other problems with the Boots, as explained in the documentation for the SP Fixpack (http://www.sorcerers.net/Games/BG2/index_editors3.htm):
- The first problem occurs if you cast another Haste or Improved Haste on the NPC whom has Boots of Speed equipped. It will also happen if that NPC uses Oil of Speed. The Boots will remove bonus attacks from those new effects, as the spells were never casted.
- Another problem comes with initiative bonus. The attack bonus is removed and so the NPC sometimes “skips” the whole round before actually attacks (effect numbers in item file match in negative manner). Besides that, such NPC can’t perform the backstabbing. The round that is “skipped” turns the NPC visible and so the backstabbing actually can’t be performed in next round.
This is solved by reediting Boots of Speed to provide 200% of original movement rate (means: these boots double the movement rate). This resolves all negative effects from Haste spell that was inside original Boots of Speed.