Depends on the session you're substituting it for. If you want to use it instead of an easy run - great. You'll stretch everything out and it'll be very good for your recovery.

If you want to use it for interval work or a hard session it depends on how good you are at swimming and pushing yourself to swim hard. I like the atmosphere of a swimming club with regular trainings in summer but i find it harder to make good use of sessions when I swim less often without the club. Apart from being pushed by others, being able to occupy a 50 m lane dedicated to going fast without having to overtake people makes the club better for hard swimming.

I'd like to swim in a club all year round but travelling across town early in the morning doesn't mesh with my study obligations whereas it's easier in summer. It gets pretty hot in summer here and then I think there would be days I can put in a much harder workout in the pool because I don't have to worry about the heat.