does the tactics trainer on chess.com actually help?

Reassess is quite an advanced book 2Q1C

Silman's endgames book is good in that it's divided into sections based on rating, so you know how advanced the material is meant to be. You're a premium member though so you could do worse than doing the lessons on this site

 It depends on how you use it. If you spend 5 seconds in each positions and choose the first move you see then the answer is no. If you spend time analyzing the position and find all candidate moves, calculate some variations you will train very important skills that will boost your normal OTB play. Personally I don't care how long time it takes me to solve a tactic, I just want to be damn sure when I make a move that it is the best move. Over time you will discover though that you starts to calculate and analyze quite fast.  

Do you think it is more useful to repeat the same theme, for example "remove the defender", 100 times in a row, or to do mixed puzzles?

I think a combination is the most useful. It is without doubt good to look at a lot of positions from the same theme but you also need to learn to sport them in random positions.  

Next time you're on Tactics Trainer, look at the comments section below the puzzles. Check out the stats of some of the people commenting. Often you'll find people who have worked on not 100's or 1000's but tens of thousands! of puzzles. You'll also frequently find that their tactics rating is not much higher than the default 1200 we all start with. In some cases even lower. Does that mean those people are dunces who just don't have a talent for tactics? No, it means that Tactics Trainer doesn't do what it's name would suggest, train you!

Most certainly. I have now done many hundreds of the tactical examples with the grade facility switched off. Not only has the exercise been extremely enjoyable, but my sense of pattern recognition, in over the board games particularly, has improved immensely - and I made no concious effort to remember them. I find they jump out at you as and when they occur. One of the best things you can do to improve your game. Thoroughly recommended.

Yes they do. Here is an example of how it has helped me. it gets good at about move 14

@Bad fish. How long did it take you to type all those yeses?

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After many hundreds of puzzles worked on, this player's tactics rating is only 1220. I fail to see the progress. But you guys just keep on doing what you're doing, at least you're doing something, right? Sorry, if you want to get much better at tactics, or chess in general, you can't just do anything with your limited time and hope that you will improve. The problem for most players, trying to improve on their own, is not their unwillingness to study but rather not knowing how to study. Before you can learn something on your own, you must first learn how to learn!

Its like football, you need to come close enough to the goal to take a scoring shot.

But sure tactics helps. If I were tactically better I would have mated my otb-opponent on Sunday, but I didn't find the move.

Yesterday my brain worked fresher, and I found the tactics. He resigned in 24 moves, my positional play was good to and the CAPS-score analyse gave me 98,66.

I do find more tactics now than I did find before I started my 270 hours of tactics training. The mating nets are more visible. The knight is easier to track.

Only solving tactics are useless. You should also learn some new patterns resulting in your rating growth. Just consistantly doing tactics and not learning from those wrong, you will never improve