My husband and I stayed at the Grand Millenium Sukhumvit for four nights in August 2012.
We chose a Club Room for our stay which included daily buffet breakfast (as noted by others, available in a smaller format in the Executive Lounge or taken in the restaurant on the second floor), wireless internet and access to the Executive Lounge for cocktails and canapes every evening. We were also given massage vouchers however ran out of time so didn't use them - a nice touch though!
Our room was spacious with a separate toilet and bathtub, and a stunning view of the Bangkok skyline. Everything was very well kept and clean.
The pool area is quite small without any of the fripperies of some other hotels (bar etc.) however the pool is large enough for a cooling dip in the afternoon and was fairly quiet while we were staying which made for a relaxing time.
All staff whom we had contact with very friendly, helpful and efficient. We left behind a bottle of bath salts as they wouldn't fit in our luggage. and received an email from the concierge asking if we wanted it returned to us via post, which was a pleasant surprise for a large hotel.
The hotel is located close (five minutes walk) to the Asok BTS line and Sukhumvit Metro station, which makes getting around Bangkok on public transport quick and easy. Taxis are also cheap and plentiful, with hotel staff willing to help in hailing, giving directions and ensuring that meters are used by drivers.
Terminal 21 shopping centre is in the same complex as the train / Metro lines and has a supermarket, chemists, newsagent and fast food outlets on the ground floor as well as multiple floors of fashion and other boutiques.
There is also an excellent laundromat called 'Mr Laundry' at 11/1 Soi Sukhumvit 23 behind the hotel if you're looking for a less expensive means of getting clothes cleaned than the hotel prices.
My only caution for those who are considering this hotel is that it is only a block away from 'Soi Cowboy', which is one of Bangkok's red light districts - fairly tame during the day but can be a bit rowdy at night. Also because of this, many of the older male clientele are temporarily accompanied by various young Thai women, something which the hotel, like most others in this part of the world, does not appear to discourage (requesting only that additional overnight guests present their identity card to reception staff).
Overall, this hotel is a solid value option for those looking for a bit of luxury in amongst the hustle and bustle of Bangkok. We would stay again.