Every year, some team is declared the "winner" of the offseason. That team is generally the one that spends the most money, but they are rarely the one that actually goes on to win the Super Bowl. Some of these teams are debatable, and I’m sure many of you will say that you "knew" they still wouldn't do well, even then, but I think it’s fair to say that most people considered these teams the "winner" of their respective offseason.
2014: Tampa Bay Buccaneers – The Buccaneers were applauded for signing Alterraun Verner and cutting Darrelle Revis. They also signed Josh McCown, gave $30m to Anthony Collins, and $43.75m to Michael Collins. They were rewarded with a 2-14 season and the number one overall pick in the 2015 NFL Draft.
2013: Miami Dolphins – The Dolphins signed Mike Wallace, Brandon Gibson, Dannell Ellerbe, Philip Wheeler, Brent Grimes, and Dustin Keller. They also re-signed Brian Hartline. Wallace, Ellerbe, Wheeler, and Grimes remain, but Ellerbe and Wheeler seem likely to be cut. Wallace is widely considered one of the biggest free agent busts in recent history.
2012: Tampa Bay Buccaneers – The Buccaneers signed the top wide receiver, offensive guard, and cornerback in free agency with Vincent Jackson, Carl Nicks, and Eric Wright. The team went 7-9, good for fourth in the division.
2011: Philadelphia Eagles – The so-called "Dream Team," with Nnamdi Asomugha as their featured star. The Dream Team finished the season 8-8 and then 4-12, before the coaching staff was fired.
There you have it. There are some exceptions of teams doing very well in free agency, such as the 2013 Seattle Seahawks, but on the whole, the team that signs the best and most expensive players in free agency tend to find that it’s fool’s gold.
That isn't to say it's the free agents that are making the teams suddenly bad. There's definitely a "correlation does not imply causation" thing going on here. Often, it's the teams that are just generally bad that have to do the most spending in free agency. The real lesson is that free agency isn't a cure all, no matter how much you have available, and the only way to make a lasting impact on a team's future is through good drafting and development.