Stayed there for 5 nights.
Room was OK in terms of amenities. In terms of size it was rather small. The balcony overlooking the beautiful Portoroz bay was particularly nice.
However, as rooms face westwards they get a lot of sun all day long thus getting particularly hot in spite of the max outside temperature in early Oct reaching into the low 20's.
This brings me to one major downside, the A/C, which doesn't produce any change in the room's temperature even after having worked in full power mode for half a day.
In October you could leave the balcony door open which would help a lot making the air inside the room breathable but was wondering what options one has, -other than changing the hotel- in mid-August when the temperature goes up into 40's and the existing A/C is useless.

I can only confirm the issue with the sewage stench. It wasn't though as present and overwhelming as stated in the previous comments, nevertheless was just as annoying when crept inside the bathroom or was felt on the sea promenade in front of the hotel.

Breakast was OK.
The outdoor bar terrace on the lobby floor has magnificent views of the bay and the nearby Portoroz city centre.

Staff is OK, though sometimes struggling to understand English being spoken at them.

The only excellent service the hotel provides is free high speed (and I do mean HIGH SPEED!) internet connection. This is way ahead of many other hotels of similar category.

The location of the Hotel is ok. It's within walking distance to Piran and Portoroz city centre (15-20 mins in either direction). There are also free busses connecting with Piran from the hotel's adjoining public parking area by the sea shore.

In other words the hotel scores average in pretty much all aspects of its offering. Pondering all aspects of its offering to me it appears as a good 4-star hotel, definitely not a 5. If you look at it this way you will not be dissapointed.
However the pricing is steep related to the offering. At 200 eur per night for a double you could get a a hotel that's closer to fulfiling the 5-star ranking than Bernardin.

Other than this, the Piran area is fantastic. The nature looks are fab. One can travel by car to Italy (Trieste and some small villages down that part of the coast) or to Croatia (especially to the beautiful little towns of Rovinj and Pola) and just as well inland to Lipica the home of famous Lipizzan horse breed and to the largest cave in Europe at Postojna.

I would return there for holiday with the family but would choose a different hotel.

The hotel has excellent location and nice room view .

Stench of sewage around the hotel and in the room was unbearable. This is unacceptable for a 5-star hotel.
Last morning, we literally ran from the room in which was unable to breath.

I agree with others - it is not 5 stars hotel - food is poor and low quality (cheep), stuff is unfriendly - when I took umbrella, a boy told me that I have to return it!!!!! So rude! Rooms are rather small and wardrobe not big enough. There is no facility for drying wet swimming suits. Carpets are old and smelly. Beaches are nice and they did a good work. Location is excellent, only 10 minutes from Piran, beautiful old and original place and 15-20 minutes from Portorose.

I have stayed at Grand Hotel Bernadin several times, always for a short trip in spring. It is in a very good location, directly at the sea between Piran and Portoroz. All rooms have direct sea view and a balcony. From the outside, the hotel is rather ugly (entirely built in concrete) but rooms are nice and very clean. The hotel has a rather new wellness center and spa, opened in 2009. I only tried the indoor pool there, which is not big but has a splendid view over the sea (especially recommended at sunset). The hotel next to Grand Hotel Bernardin has a very large indoor pool area which is suitable for children and can be used free of charge, if I remember correctly (the two hotels belong to the same hotel group.)
I only had beakfast buffet at the hotel which is good but not overwhelming. I din't like the coffee, which you have to take from a coffee dispenser. But they offer a wide range of fresh fruit, and the selection of cakes etc. is also good. I have never tried dinner at the hotel as there are a lot of very good (fish-) restaurants in Piran. It's a 15 minute walk from the hotel. I would definitely recommend to eat in one of the restaurants there at night. By the way, Piran is one of the nicest spots at the Adria, small venetian town, very pittoresque!
Hotel staff is in general very helpful and nice. Reception staff speaks English and German, and I think also Italian.
If you want to go to the hotel in summer, consider that there is no sandy beach. You can lie down and have a swim directly in front of the hotel, but it's more a concrete boardwalk than a beach.
I would recommend the hotel for a trip to the Slovenian coast but I would give it 4 stars rather than 5.