The trend in that graph is incredibly promising either way.

Promising in what way? It shows that Bitcoin is gaining more popular attention, which is translating into increased sales for Coinbase.

The issue is that Coinbase has some serious flaws that make it immensely unappealing to any serious Bitcoin enthusiast.

If you go to some Bitcoin communities and search Coinbase, you'll just see an endless stream of negative opinion.

(Reddit search for Coinbase, first 5 results are complaints: http://www.reddit.com/r/Bitcoin/search?q=coinbase&restri...

The same search on bitcointalk.org shows similar results )

It'd be more promising if they showed returning buyer stats; i.e., those who bought a BTC and then returned the next month, let's say, to buy more.

I'm kind of disappointed in Coinbase. I really hoped they would be a serious competitor to Mt. Gox, which would be a great thing for Bitcoin.

Not to my eyes. After the early April surge, you could easily make a case that it's shrinking. At the very least, the peaks camouflage much slower than 15x growth.