As mentioned above, I was here recently and was not impressed at all
Here's my pros and cons:

PROS

-The executive club lounge is on the 1st floor next to the breakfast area and is open 7 days a week until midnight. Evening food and alcohol is served from 17:30 (5:30 p.m.) to 20:00 (8 p.m.). The offerings are decent with 3 (small) warm options, and various cold options, including dessert. These offerings change very night. However I'm not sure if it can substitute for a full dinner. At all other times, the lounge is limited to a few soft drinks and a coffee machine.

-As the lounge is right next to the breakfast restaurant you are not limited to only a free breakfast in the lounge only as other JW Marriott might require. You can have the full buffet and bring into directly to a lounge table.

-Breakfast buffet is more extensive than other hotels I've been to, with a lot of different vegetables on offer. Breakfast is served until 10:30 a.m. on weekdays and until 11:59 a.m. on Saturdays and Sundays.

-Turn down service is done every night without asking.

-Hotel offers elite members a coupon for a free dessert at the JW steakhouse or a free drink at the bar.

-You can open the windows here for fresh air, even on the higher floors.

-Quiet surrounding area, however see "cons" below

-Late check-out granted without fuss.

-Permanent taxi rank just outside.


CONS

-Really old rooms and decor from 30 to 40 years ago. It's all clean and fairly well kept, but it's like a trip back in time 35 years, and smells musty, especially when you first enter your room. Almost all these rooms have the old bathtubs and NO walk-in showers.

-Only the rooms on the 6th floor have been renovated and refurbished, however they still have the old bathtubs, and NO walk-in showers. The renovation was only done halfway, as the front room door, bathroom door, wall closets, safe, bathroom sink, and mini bar area, are all EXACTLY the same as the older rooms from 35 years ago. Even the desks and bedside cabinets are exactly the same, except that they added a layer of marble on top to make them "appear" more modern. The main change in the renovated/refurbished rooms is that the old smelly carpet has been ripped out and replaced with marble flooring, the walls have been repainted bright white, the curtains have been replaced, and 2 modern (yet strangely uncomfortable) single seater sofas were added by the window.

-On the 6th floor during the renovation they have put in marble hallways everywhere, which causes a lot of echo noise. If you are in your room you will hear it very clearly every single time someone passes the outside hallway with their luggage, or the the cleaning staff pass with their trolleys.

-Long lines at check in with no priority lane for elite members.

-No upgrade offered despite being titanium ...had to ask 3 different members of staff on 2 different days, as they repeatedly kept telling me to ask later, Upgrade to a basic sized 6th floor renovated room was reluctantly on day 2 of my stay.

-There are no restaurants or shops within a 10 minute walk.

-There are no metro stops or stations within a 10 minute walk. Walking back and forth 15 to 20 min to the "Potsdamer Platz" station gets really old after a few days, and it awful if its raining or snowing. I'm not sure I'd want to do the walk late at night ether.

-Awful service at the JW Steakhouse restaurant, despite there being 3 or 4 different waitresses and the restaurant being half empty. It took 20 minutes for our table to be cleared after finishing eating, and another 20 minutes just to get our bill after asking. If that wasn't bad enough already, to add even more insult to injury, the next day at check out I discovered the restaurant got some receipts mixed up, and had also charged some alcoholic drinks onto my room which I never ordered. Obviously must have been from someone else's table.



Conclusion:

Overall I found the hotel very disappointing, and would not return even if the rate was cheap. It just feels like a very old Sheraton that maybe was considered luxury hotel 35 years ago. (On a more general note, the entire "JW Marriott" brand seems to this way, no matter where I go, as this is my 4th (and probably last) attempt at staying in one of the "JW" branded hotels across the world. The "JW" properties all just seem so out of date, and don't feel 5 star at all)

Rates here can fluctuate greatly from as low as 120 euro per night, to over 350 euro per night for basic entry level rooms. You may be tempted by the cheap rates, however you will not be getting a 5 star luxury experience by any means, especially when you see your room (renovated or not) with an old bathtub, get lacklustre service, no upgrade, and come to terms with the poor location.

Staying at the Moxy (Ostbahnhof), Courtyard City Centre, the Westin, or the other Marriott at the Potsdamer Platz, ALL offer much more modern rooms with walk-in showers (even in entry level rooms) and ALL have metro/U-Bahn/S-Bahn stations just outside the door.

Also, if the club lounge is a motivating factor for you, the Marriott at the Potsdamer Platz has a better lounge in my opinion with better views and more food on offer. Even the club lounge at the Sheraton Grand Esplanade (another very old fashioned property) had better warm food offerings in the evening.