I have a 3.5 Tessar version and it is very sharp and contrasty. In fact at f8 it's sharper than my Blad 80mm lens. I love this camera and when I just want to go out and do some fun stuff but want good quality I use the Rollei. I've made wonderful 16 inch prints with it as well.
With all due respect quite frankly you'd have a hard time finding any lens made in the last 70 years or so that would be a bad performer stopped down to f/8. If f/8 was fast enough for everyone all lens would be built to that spec and would perform beautifully. It's the more open performance that people often seek (faster lens, more narrow DOF) and thus the desire for perhaps a f/2.8 lens.
Now that said you really do need to ask yourself how often you will truly be shooting wide open and is that extra weight worth it? I have a 2.8E Planar, a 2.8C Xenotar and a 3.5E3 Xenotar. I got all three of these for very, very good prices ($160 USD, $275 USD and $175 USD, respectively) by just being patient and jumping when I saw them, but all three needed either a CLA or at a minimum a focus adjustment.
The Planar is my fav' for B&W work but not in a way that can be put into words. My example is actually a unique, quirky camera (found in an alley shop in Beijing of all places), ugly and beaten as heck, with a decent scratch on the front element and seems to have little to no coating left on the lens as well. This could be the reason for it's "charm" I find in it's B&W images. That said though the shutter just jammed for the 2nd time and it's off for another fix. My 2.8C Xenotar looks as if it was kept in it's original box and rarely used it is so clean and pristine. I love the images I get from it (they may actually be a tad sharper than the Planar, but I'm not one to shoot Air Force Test Charts and get out my magnifying glass) but as mentioned I prefer the Planar for my B&W work. My 3.5E3 Xenotar is a another beater I got for a song and it needed a serious CLA and adjustments and it's bent in side to be hammered back out again. But now it is giving me simply gorgeous images.
I have found that while 2.8 vs. 3.5 will give you some narrower DOF and of course allow you to shoot in lower light levels the difference is truly minimal.
I'd say find one you like in good condition and give it a try. If you buy right you can usually sell at the price you paid should you decide to try another. Just remember that the cheaper it is the more likely it will indeed need a $175 USD (minimum) CLA to get it working correctly.