As someone who's stayed at this hotel enough times over the past 14 years to be practically a resident, I'll confirm that the previous posters in this thread have it spot-on right. I'll just clarify that upgrades are usually limited to non-deluxe-to-deluxe, or deluxe-to-balcony, whereas jumps to suites are unusual unless based on a travel agency or personal relationship.
The rooms have been refurbished over the last couple of years. And the new general manager has introduced a lot of guest-focused and guest-friendly innovations that help make the hotel feel like an urban resort or even a home away from home (as it is for me). Structurally, though, not much has changed with respect to the rooms over the years.
But the surroundings have changed. Development on the Peninsula's side of the river is gathering a lot of steam. There is now a neighborhood with visitor-friendly outlets you can go to on the Klongsan side, with another major development underway, so the Pen is no longer as isolated as it was in the past. And there is more development and things to see downriver, so rooms on the downriver side of the building (suite -09 and rooms -10, -11, -12) now have better views than they used to, though the view upriver to Wat Arun, the Temple of the Golden Buddha, and the Grand Palace complex and Wat Pho is still more desirable.