It's flash capabilities are not "not all that great", they are non existent.
It depends on how much you want a severely limited optical master. Personally, and I own both, I'd recommend spending a little more and getting a secondhand ST-E2 (around $110), it isn't much bigger, is much more "powerful", it has a focus assist, and you don't have to do everything from the camera menu.
I put in some new panasonic batteries and while I wouldn't say the batteries were great... they took their sweet time to recharge. I play around with the 90 ex and my two 580 exii's in slave mode and I like the results... I played with the ratios and I do like having a dual flash set up... but I would happily sell my 90 exii for a $50 if anyone wants to give me that much (via Craigslist)... it is ok... but there were enough times where I was annoyed because I assumed the flash would be ready and it wasn't.
+1 I owned a 90EX for 7 months and recently sold it for $40 on eBay. Since it can't swivel to use for bounce flash, I only used it as an optical master, and it didn't perform well in that role.
As an optical master it works, but I found the recycle time to be incredibly slow. In the studio I was missing lots of shots with our toddler while waiting for it to recycle. Also, since its fixed, you are forced to keep your slave flashes in front of you to trigger them. I recently purchased two 600EX-RT's and now use one of those as my master, so the 90EX was just gathering dust.
I recommend spending a little more money on a more capable flash.