Yup... Editing songs down is part of the mastering process sometimes too. Althought you'd probably get better results these days editing it digitally yourself beforehand.
Back in ye olden days of 45's and 33 1/3 LP's, the 45's were always edited down to about 3 minutes and pushed as hard as they could get them.

Pro mastering is definitely the way to go but you have to weigh the costs against the return. If you can afford it, then hell... go for it.

Mastering involves editing your songs for time, putting beginnings and ends on them, fading out.. etc.. Plus (using outboard profession top-end gear) EQing to bring out the best of each mix, Compressors and limiters to make all the songs of equal volume and sound regardless of when or where they were recorded. Plus a high end listening room that the engineer's worked in long enough to know exactly how to get the best sound.
As with anything though, you get what you pay for. You're basically paying for the mastering engineers years of experience.

If you're just releasing a home-demo CD though... it's a tough call to make.
Again, if you can afford it...what the hell. But if you're selling your own CD over the internet and moving maybe 100 copies if you're lucky.. it's a good idea to look into using software mastering and build up your experience yourself. You'll never get the high-end quality that you'll get with outboard high-end gear, but you can do a reasonable facsimile of compressing and limiting, EQing and all the necessary editing.

If your CD starts to sell really well, you'll be in a position to get it signed with a label, who'll then pay to take your original mixes in for proper mastering later.
(and pay for distribution and all the nifty advertising stuff. All against future royalties of course...blah)