is anyone up for some information exchange about companion planting?

because i generally 1: can't be bothered 2: am both ill-informed and
disorganised, i don't companion-plant (also, wouldn't it mess with rotation
systems? that might be a different issue though) it's not something i
pursue. however, for 3 years i have put basil with the tomatoes in good
faith because EVERYONE says this is a good thing.

however, over the 3 years i've noticed that the basil doesn't come on if
it's with the tomatoes - the plants are always small (although the tomatoes
are happy). last year i thought the basils were probably too shaded, so i
gave more space this year, but the basils are still small & flowering early.
my other (seperate) basils are as big as i would expect, though (so it's not
dud seed or anything like that).

while i don't normally have any tomato diseases (so far - i am quite a
newbie gardener though) i can't be sure if this is because of the basil, or
if they'd have been healthy anyway (as, after all, tomatoes really aren't
that difficult, are they?)

so in a nutshell, i'm going to give up on this particular arrangement.

i have one tomato plant, though, that was planted quite close to spring
onions, & it's a tiny plant. i'm taking out the spring onions as we need
them, & the plant's finally getting a bit bigger now. i wonder if the onions
are bothering the tomato plant. the other, self-sown toms which are various
places are just dandy, despite considerable neglect AND no basil nearby (and
no staking, and little watering, etc).

does anyone have any comments about good or bad companions? have any of you
investigated this seriously? is it even worth pursuing? if anyone's found
any really excellent companions, i'd be quite interested, because i'm
starting to think it's just one of those things people say because everyone
else does, but has rarely been investigated thoroughly & scientifically.
thanks!!
kylie