I went to Slack's last night with a bunch of girls (all gay) and my husband and it was a is a pretty good time. It's a small bar at the corner of Church and Wellesley with a mix of young and old lesbians with a few guys sprinkled in the mix (FYI there is an enforced 2 girls for every one guy ratio at the door).
We strolled in at midnight on a Saturday with no cover and no lineups. The drinks are moderately priced (about $5-$6) and I didn't wait long at the one long bar to get served but I found my rum and coke a bit watered down. I am usually not a fan of top 40's music but there was a good mix of old school hip hop like Ol Dirty Bastard!
As the best (and possibly only all lesbian bar) in the village it was a good time and safe space for me and my friends.