Portobello was actually my least favorite market as it was the most commercialized and the one with the most tourists. Plus, Portobello tends to go on and on and on -- and worst of all, there are no food stalls like you find at some of the other markets on the list.
The few times I went, I noticed a lot of antiques -- antique jewelry, antique silver, antique cigarette cards. I bought a couple of old Babar prints and noticed that many of the stalls were selling other kinds of prints -- like vintage advertisements.
Portobello is only open on Saturdays. Get there early to avoid the crowds of tourists and make sure to look at the beautiful houses of trendy Notting Hill.
Although this isn't my favorite, I'd still recommend it to tourists since Notting Hill is an interesting area to visit.