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Downside:
1. Could get banned at all (from review process)
2. hard to get out (will lose traffic for a while if you quit until you get the natural rankings back.)
3. Could be expensive (you pay for click now as well as for URL)
Overture OFFCIALLY DENY any placement improvement for sitematched sites, but unofficially there is A LITTLE BIT of improvement in the ranking since Overture approved those pages manually.
The biggest advantage of SiteMatch is you get your pages into the index quickly. Minimal waiting. Then you can use your SEO skills to constantly change on page factors and see what changes these have on your rankings.
If it would be more desirable to start paying for traffic, then SiteMatch is the way to go. If Y! doesn't currently index the site for some tech reason, then SiteMatch may help to get around that.