Skydeck has very little impact on battery life. The sync client may be running in the background all the time — it depends on your phone — but it uses very little power, because it only communicates with the server briefly, when you use your phone.
There’s one exception. If you have a large list of contacts (more than 500), you may drain the battery the very first time you sync your address book to Skydeck. After the initial sync, Skydeck only needs to relay changes to your contacts, so it won’t happen again.