I'm facing the same decision as the O.P. I have been using Pro Tools 8 for four years now and have really enjoyed it. It has been rock solid for me, even running Windows Vista. However, it is no longer being updated and I have outgrown my front-end setup (small Soundcraft mixer into an M-Audio pci interface). After the holidays, I'm upgrading to an Allen & Health ZED 10FX usb mixer and have decided to upgrade my software as well.
Initially, I was only considering PT 10. However, a number of the major DAWs have seen recent upgrades, so I have decided to demo every one I can to see which I prefer. I intend to track and mix a small, test project (Elvis - Burning Love!) to see how each fits my particular needs. I am underway with the first couple (Sonar X2 and Studio One 2.5) and am going to follow through with Digital Performer 8, Reaper 4, Cubase 7 (once available early next year) and then, finally, Pro Tools 10. I'm even going to give FL Studio, Reason and Ableton Live a glance, though I already know that none of those are the DAW for me. I'm old and not nearly cool enough.
I was a Sonar user before Pro Tools so I sort of knew what to expect with X2. I am enjoying some aspects of it, including the interface and integrated channel strips. However, I have been underwhelmed by the MIDI editor and the lack of flexibility with the mixer (can't drag channels to a new location or change their color, unless I'm missing something). I think the interface is too busy, with too many infrequently-used options on the mixer channels.
I only started digging into S1 last night and I have to admit, I am *really* liking it. Clean and efficient and responsive. Very nice, handy shortcuts. I love the on-strips controls for all bundled plug-ins. From the time I double-click the icon, it loads in about 0.0235 seconds. Pro Tools can take upwards of a minute sometimes. Just getting started with this one, but it's made an excellent first impression.
Of all of my options, I am most excited to try Cubase 7. I love the look of the new MixConsole! Of all my gripes with PT (except the lack of Track Freeze), it's the constant opening and closing of plugins and filling up the insert window with plugin buttons that annoys me the most. I'm very stoked to try mixing with the inline strip. I am a programmer by day, and this software has the feel of "clean, uncluttered, new code" to me, unlike the bloat that accompanies many DAWs.
If this thread survives, I'll update it with compares & contrasts of the various DAWs as I test them, in case anyone stands to benefit from my experience.