Obviously, it will need to deal with the humidity at the launch site, but the boost from being the equator is significant.

Soyuz rockets set to blast off from Kourou

By Steve Nichols

Arianespace has signed a contract with Roskosmos, the Russian space industry, to buy the first four Soyuz rockets that will be launched from the Guiana Space Centre (GSC) in Kourou.

Jean-Yves Le Gall, chairman and CEO of Arianespace, and Anatoly Perminov, managing director of Roskosmos, signed the contract at the show, and confirmed launches will start in 2009.

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The medium-launch Soyuz fits in well between the existing Vega light launcher and the heavy lift Ariane 5.
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it is expected to become the most cost-effective path to orbit for commercial satellites in the 3 tonne class.

Very true. The Russian boosters are very mature technology, and most of the costs of their development have been amortized long ago.