Collisions are integral to the way Ethernet works, so I suppose you could say they are good, because if Ethernet didn't work most of us would be out of a job!
A big, in-depth discussion is probably not the right thing to do in a post, but if you want more (much, much more) detail, have a look at the excellent Ethernet tutorial under the Networking menu at the top of the Firewall.cx site page
However, to try to directly address your point, what you want when using Ethernet is a collision domain that is not too big. If you have too many devices and/or too 'long' a segment then you get too many collisions occuring and so get worse performance. Hence the use of bridges and/or switches to break the network up into smaller collision domains
