The reason I booked this hotel is the cool design and the not-so-bad reviews the hotel has on internet forums like this one.

I stayed at the hotel for a week and here are mine impressions:

The good:
Clean rooms, nice furniture, cool and inspiring design of the hotel.
Friendly receptionists, at least the two persons that I have been dealing with.
Free internet trough LAN-cable. (Availible at the reception for 20 EUR deposit)
Phantastic glass wall with an outside view.
Electrically operated curtains that block sunrays or curious eyes.
Congestion in the sink - the personel fixed that same day.
Armchair leg broken(!) - the personel fixed that same day.
Very close to the subway and a big shopping place.
Very close to the spectacular Torre Agbar - cool tower that can easily be seen from an airplane while landing or taking off.
Great elevators.
It is not possible to crank up the volume of the TV to very high levels - no disturbance from deaf neighbours.

The bad:
Very noisy road and a adjacent construction of a big building.
Breakfast not included - in my country good breakfasts are included as standard.
The view is so-so from some of the rooms. Mine view was not phenomenal despite I stayed at 7:th floor.
Not very close to the centre of the city from the hotel, but possible to walk - approx less than 30 minutes.
The floor of the bathroom gets wet when showering.
No equpment to make tea, neither equpment to iron.
The pool on the roof was closed - come on it was only October! :)


Other:
It is possible to see from the room in to the shower. This might be a problem for people with kids. Great for couples that find each other attractive, other couples beware. :))))))))

My wife and I booked 3 nights at the diagonal via Easy Jet's own website. I was keen to see the hotel, as the previous reviews had left me feeling curious.
In appearance, it is very stylish, very cool. Lots of modern decor and an upmarket feel to the place. I found the staff to be pleasant and generally helpful, although perhaps not at the pace that I am used to as a Londoner. The lock broke on our room one night as we were leaving the hotel to eat, and it took around 45 minutes for it to be fixed. Not a complaint really,more a small inconvenience.
The rooms are very well appointed, ask for a view and you will get one. The Sagrada Familia is visible, and there is generally a good panoramic view of Barcelona. We were on the sixth floor and could see loads. The bathrooms are very nice, our bath leaked a bit but nothing dramatic, good toiletries.
You are very close to the metro, you can be in the town centre in 10 minutes. We didn't eat in the restaurant, seemed a bit pricey, but there is the shopping mall across the road with a few different cafes and fast food joints for breakfast.
So, would I recommend the place? Yes, but not at their listed room rate..if you can get a good deal..definitely. Oh, and the beds are a bit hard.

Stayed at the Silken Diagonal for one night before embarking on a cruise. Taxi from the airport to the hotel was 26 euros and took about 15 minutes. The hotel is very modern-the lobby bar and reception area is lovely- 70 style chairs and furnishings, fab! Check in was fine- the staff spoke enough english to check us in smoothly. Our room was on the highest floor and although the view wasnt exactly picturesque, it was lovely having a huge floor to ceiling window. The room itself was modern and clean, with enough room to move around despite our two big suitcases. The only strange thing was that the bathroom 'wall' is a kind of frosted glass which means that anyone in the room can see you in the shower- fine if your staying with your partner but not so great if you were sharing with a friend or your mum!!We went up to the rooftop bar for a drink but as there was a private party going on we couldnt stay long. The view at night over the city was lovely.
The hotel is located just outside Barcelona city centre- there is a subway station a 2 minute walk from the hotel and the tube only takes 3 or 4 minutes to get to Las Ramblas. There is a large mall opposite the hotel that has quite a few shops and several places to grab something to eat.
All in all, a good hotel and I would return went next in Barcelona.

Agree with most of the reviewers - the staff's command of English was spotty.... some were great and others not so much.

Perhaps after a two-week cruise, we were spoiled, but assistance seemed reluctant.
1 - The lights don't turn on unless you put your room key in a special slot - Were not informed - had to get the front desk to help.
2 - Directions to the Metro was vague, but there were signs on the way
3 - Requesting best way to get to airport via Metro was met with much confusion

The Metro works well, but it takes about 3 transfers to get to the airport from this hotel. Additionally, the route to the underground was through a deteriorating neighborhood - lots of trash and grafitti. We decided that with luggage, it would be better to take a cab. The hotel arranged the cab and like a previous reviewer noted - about 30 Euros and 30 minutes to the airport.

The rooms were futuristic. We really loved the double curtains that completely darkened the room. When we rolled them up in the morning, we were surprised how bright it was outside. They were very effective.

The toilet has a door, but the shower is visible from the bed - just a skimpy curtain and glass. Not an issue for a couple, but a family with kids might have concerns.

The breakfast was outstanding - well prepared, plentiful, and varied. Definitely not your typical continental breakfast.