I had a lot of time to compare the HD 650 against the T90 (I even wrote a bit about it here), then used the T90 full-time for a few months until I upgraded to the HD 800.
In short, the HD 650 has a very weak high end, which makes it sound "warm" or "laid back" at the expense of crispness and detail. Some people love this sound, but I don't. I was also disappointed in its tacky, dated sparkly-plastic shell, although it's very comfortable.
The T90 has extremely impressive crispness and detail — in those regards, I'd say it's nearly indistinguishable from the HD 800, and may even be slightly more crisp. The tradeoff, and why the HD 800 can command nearly 3 times the price, is that the T90's high end can sound harsh with some recordings, especially mediocre '90s and '00s pop and rock recordings, which were often mastered with too much treble. (The HD 800's upper end is more even, and retains most of the detail and crispness of the T90 without being harsh.)
But the occasional upper-end harshness is the T90's only significant downside. In most other respects, it's extremely competitive with the HD 800. It's even more comfortable — my #1 headphone for comfort, actually. And it's much easier to drive than either the HD 650, HD 800, or HE-500 — you don't even need an amp with the T90 unless you want it really loud. It can be adequately driven from an iPhone.
I haven't tried the HE-500, but from what I've read, I'd guess that it's better in the lows than both the T90 and the HD 650, but isn't as crisp in the highs as the T90. The HE-500 is also significantly heavier, and many people report that it's less comfortable as a result. (Keep an eye on the new HE-560, which reportedly has improved highs and less weight.)
I'd confidently recommend the T90 to anyone who likes crispness and treble. If you don't — if you want the "warm" sound of a rolled-off high end — it's not for you. It matters what you listen to: all live concerts and well-produced albums sound amazing on the T90, but some rock/pop studio recordings between about 1998 and 2010 are iffy.