Opinion | Fernando Alonso may be mulling retirement as McLaren struggle

Formula One is back in its traditional heartland this weekend - Europe. Barcelona will be the fifth race in a 19-date calendar and it is a good time to take stock.

For some there is a lot of work to be done, and being back closer to the factories will be a huge relief. For others there is already good momentum.

At the front of the grid, Mercedes are where we all expected them to be, and Lewis Hamilton is looking every inch the dominant champion within the team.

The big surprise is just how good Ferrari have been. Even those in the paddock have been taken aback by their form.

A look at the world championship will see why Mercedes are treating the rejuvenated Prancing Horse with respect. Although Hamilton leads the standings, Rosberg is just one point ahead of Sebastian Vettel.

A glance at the constructor's standings will also confirm just how well the revival for Williams is going. In a way it needs to, with the team recently announcing losses for 2014 of £40 million (HK$469 million). That had a lot to do with their ninth-place finish in 2013.

Their driver Valtteri Bottas has been relaxing ahead of the Spanish Grand Prix by going clay-pigeon shooting. He may also be taking aim at a drive for Ferrari. Reports say Ferrari boss Maurizio Arrivabene is mulling a move for Bottas in 2017.

So far, so good for those impressing in the early part of this season. But perhaps the most interesting stories in the sport lie with those who have been struggling this season.