Back from 2 weeks in Costa Lago about a month ago with my family and an other 2 families. The hotel is decent and the food was impressive especially in the first week (got a bit repetitive after 2 weeks) but there was always something to eat. The set meal times for Dinner are not great but as the hotel is so busy you can see why. The entertainment was basically non existent so I think this something RIU will look into.
One problem was that the bar staff seemed to want to serve "older" or other non british customers before myself and friends regardless of the fact they knew we where waiting longer (sometimes upto 5 minutes). I found this quite rude and I would advise you nip it in the bud early or you will have to suffer like we did. Afterall you are on holiday and don't want to moan but it does get annoying after a while!
My most important information for you is to NOT use the Tour/Exchange shop just over the road from the hotel. We heard numerous stories of the buses not turning up and people not getting a refund. I was also informed the staff were lowering the exchange rates just when your changing money. I suggest you go further along the beach front to find a different tour/exchange shop if you are looking for excursions or to change your money.
I would also advise not to go to the bar on the corner of the road leading upto the hotel (next to the mini supermarket) as the man is extremely rude and his beer is flat/warm. Your better walking along the front to Kennedys.
All in all I would go back to the hotel but maybe for a week next time