So Google officially confirmed that HTTPS is a ranking factor about a month ago. A few days ago Search Metrics released results of a study showing there doesn't seem to be any discernible correlation between using HTTPS and search rankings.
This is not really surprising as Google stated that HTTPS is for now "only a very lightweight signal — affecting fewer than 1% of global queries, and carrying less weight than other signals". It seems that Google wanted to include it as a signal only so that it could encourage adoption, and not to actually impact their search results.
There are good reasons to think the SEO effect of HTTPS as a ranking signal will be negligible in the foreseeable future. Until the average visitor starts to care about visiting secure websites only - Google can't put too much emphasis on it. After all - if the best search result for a query is not delivered via a secure protocol, should it be shown in the search results lower than an inferior resource delivered over HTTPS? Our thinking is - no, at least not until SSL adoption grows significantly.
So should you switch to HTTPS now? Maybe. But do it because you care about your visitor's security, not for SEO reasons.