Sense works great for me. You have a device with issues, but most of us do not.
Sense is what HTC branded devices are all about. The option is to buy a device with Sense or buy something else. Similarly Samsung devices have their "Touchwiz" interface. It's part of their brand. Realizing you don't want a device with HTC Sense is the kind of thing it's better to make up your mind about within the return window for a refund. You are correct though, in saying you probably should never buy another HTC device, because Sense is not for you.
Perhaps the replacement will work better for you.
And you can always put on a 3rd party interface such as Launcher Pro or ADW Launcher.
If you don't use the replacement Sensation at all and they send you a "brand new" one. If you don't open it, you could sell it on Ebay for a nice price and use that money to buy something else. The choice of a few days without a device to transition to something else, versus having something every day and possibly being unhappy is yours to make.
I am happy for you that you are not having any issues with Sense. However, I know I am not the only person who has these issues. HTC has been dealing with this since the phone came out, and if you google this you will find out how many people are having issues. I think that for HTC not to take responsibility for messing up Android is just outragous.
Also - HTC clearely told me not to use Launcher Pro, or any other replacement launcher since it will cause further issues.
I do not know if the ICS/Sense3.6 update would resolve some of these issues, but I'm not counting on having a timely update from T-mobile.
I also doubt HTC will offer much support to phones with Sense 3.6 since they are really pushing Sense4.
I am here to just warn any potential buyers. Most likely they would suffer the same issues.
As I said "most" Sensation owners do not have your issues. There are some of any device owners that have some kind of issues. But most simply don't.
I can't say HTC messed up anything, because your's is not the experience most are having.
Far as a 3rd party launcher. Worst case if a 3rd party launcher caused issues, you would be to uninstall it or maybe have to perform another factory reset. You would not end up any worse off than you feel now, since you would still be able to undo anything installed. You could try ADW Launcher's free version and see what happens.
The ICS update just might resolve issues for you and that update will be out in the next few weeks or so.
You are under warranty for 1 year from the date of purchase and so HTC will continue to offer you support.
Also, when first buying a device, you have a 14-30 day return window for a full refund of your purchase price. This is the time to check out a device and determine if it is right for your needs.
I have to agree with everything darnell said but I also wanna add something... search all device forums. You're going to find issues with EVERY device that comes out. If it's made by Motorola, it has problems. If it's made by Samsung, it has problems. If it's made by HTC, it has problems. Hell, if it's made by Apple, it has problems. If it's made by ______, it has problems.
Do a warranty replacement, be nice to the rep you're talking to, and maybe if you ask nicely, they'll waive the replacement charge for shipping since they "mistakenly sent you a defective unit".
Don't listen to HTC's advice on this issue. I don't think there's hardware defect in your phone either. They are simply reluctant to admit that their proud Sense feature is far from perfect. Sense takes too much resources and causes 'reload' too often.
All you need to 'remove' is HTC sense itself. After 4 months ownership of Sensation 4G, I replaced Sense with ADWLauncher Ex and Beautiful Widgets. It does take time to configure and have youself adapt. After that, I am much happier with these replacements than with out of box Sense.