Jack's is a Portuguese 24 hour bakery.
The pastries are above average with the custard tarts being a highlight.
I would definitely AVOID the "chocolate" croissants. They fill them with a kind of chocolate pudding (instead of actual chocolate) and I find the consistency and flavour just gross, it's like filling a croissant with jello-o pudding.
Aside from that thought I'd say it has some of the best pastries in the area. The cheesecake is very good and they have huge selection.
As the place is very Portugesey some of the staff don't speak much English and may not be able to explain what something is very well if you ask them any kind of complicated question. There are often lots of Portuguese locals inside watching soccer or reading the paper etc.