Know what you're getting into. Much of the code at JHC is written in a language called RPG. RPG was developed by IBM in the 1960s on punch cards, while it has seen improvements in the last 50 years it is not a modern language. If you join JHC as a graduate, you will be expected to learn and use this language full time.
Do not expect to be taught java/C#/scala.
If you decide to leave JHC and want to remain a developer, your prospects will be limited to either one of JHC's clients; starting from scratch and retraining to another language; or trying to find an RPG job somewhere else (do some googling - there are jobs out there, but not many).
Pay and bonuses
Expect to be paid on the lower end of the market rate. Bonuses are also low - in the few years I was there I got between £500 - £1,500 after tax.
Exams
Expect to have to do some banking exams (you only have to pass one) which you will never use again. This might make sense for people dealing with clients to do, but isn't helpful to anyone else.