The viewing platform is not right at the top of the building but, at 30 floors up, it is high enough for good views of Warsaw. We went up after dark on a Sunday and there wasn't much of a queue for the lifts. But remember that if there is a queue going up, there will be one coming down too! It may be possible to 'skip the line' if you have a Warsaw Card (and want to make yourself unpopular). There's a good view of the lights at night, so long as the cloud base is high enough, and the building is open until 20:00 - Mondays too, unlike many Warsaw museums.
The rest of the Palace is worth some exploration - it is a soviet-style monolith with iron curtain-style manning levels: commissars, lift operators, security guards etc. The levels of labelling are a bit sparse though - we tried finding the Tourist Information Office, which is on the opposite side to the main door, and its not well sign-posted at street level (It does have its name in big letters at a height of 15 metres or so above the pavement). It's also possible to find Iron curtain car hire around here - Polski Fiat 125s and 126s.