There are lots of pics of my Guineas on the My Coop link under my avatar pic over on the left of this post.
Guinea Hens do lay well, usually an egg a day but will skip a day every 10-12 days (they are seasonal layers tho, Spring thru Fall only). The eggs are small... 2 Guinea eggs = 1 large chicken egg. They do not typically use nesting boxes and will prefer to nest outside in the brush, tall grass or other well hidden private places if given the choice... so you may be egg hunting a lot. Young Hens tend to drop eggs where ever for a few weeks until they figure out what's going on and decide to find a nesting spot. If you collect eggs daily they will keep looking for new, more private well hidden spots, but if you let the eggs pile up they tend to go broody after they have laid 20 or so eggs. They don't always stay broody tho, especially if the nesting spot isn't private enough to their liking.
You might do ok with just a few Guinea Hens in with your chickens, but if you have any male Guineas in the mix there could be aggression directed at your chickens.