The Astros bullpen is by far the worst in the league. As of last night, their ERA of 5.23 was last by nearly 3/4 of a run. But, historically, a 5.23 bullpen ERA is not that rare. Since 1916, a bullpen has put up an ERA exceeding 5.20 110 times, over one per season. However, by one stat, at least, the Astros bullpen is far and away the worst in history. Or at least since 1949.
sOPS+ is a stat that compares a player's or team's performance in a particular split to the league wide performance in that same split. For pitchers, 100 is a league average OPS against, so anything over 100 means the pitcher allows a higher OPS against than league average in the same split. Here are the top 5 worst bullpen sOPS+ since 1949 (which I believe is as far back as the stat goes), with the corresponding ERA's.
2013 Houston Astros 143 (5.23)
2007 Tampa Bay Rays 139 (6.16)
1987 Boston Red Sox 136 (5.45)
1950 St. Louis Browns 133 (6.81)
1975 Chicago Cubs 132 (5.22)
There you have it. The Astros bullpen ERA is abnormally high, but not unique in baseball history. However, they have allowed a higher OPS, relative to the league, than any other bullpen since at least 1949. Well done, guys. Well done.