Thalys Reliability

I've taken Thalys trains a number of times and it's been fine...mostly on time, some slight delays, and one long delay due to an incident on the track ahead. However I manage my expectations and don't count on any form of transportation to be 100% reliable regarding arrival time. So just don't book any nonrefundable tickets based on a specific arrival time and you should be okay.
One tip: I always take food with me on the train - breakfast pastries, a lunch sandwich, etc., and a drink. I've been on various trains that were supposed to have food, but either the food car was mysteriously closed, had a long line, or was too many cars distant from my seat. There are good food options at Gare du Midi where you can pick up something for your journey.

Much cheaper than what? Thalys is by far the most convenient option from Brussels to Cologne !

cheaper than Dbahn

Thalys is by far the most convenient option from Brussels to Cologne !

Is it really by far more convenient than the ICE? Personally I'd say that the ICE has a small advantage as it stops at Brussels Noord as well.

Doing a quick check for early August, it seems to depend on the train/class, but mostly ICE is cheaper.
I agree with Badger that the fact ICE also stops at Brussels Noord can be convenient.

Oh, sorry, I was not aware that ICE were also running to Brussels (I thought Thalys was the only high-speed option on that route). In that case, ICE or Thalys male very little difference, so I would pick the cheapest.
I would avoid Brussels North station at all times if possible, though. High proportion of shady characters there.

I agree about Brussels North.

Of course a train change is extra effort, but note that all International arrivals at any of the three Brussels stations include the right (with the same-day International ticket) to one local rail (NOT metro subway) trip to the other two, runs many times an hour. I don't think I've ever been on a platform at Brussels Nord, but I don't expect any train station to be a warm, fuzzy, welcoming place.

if you have a particular train in mind, you can look up on time percentages for the last 30 days online- websited is called Zugfinder.

I think Balso and Nigel mean Brussels Midi (South) re shady characters. Brussels North is a much smaller station, and much easier to use than Brussels Midi. Brussels Central, despite the name, is only "central" because it is between the two other main stations (and is closer to the touristic center of Brussels), but not many (if any - would have to check) international trains stop there.

Oh, sorry, I was not aware that ICE were also running to Brussels (I
thought Thalys was the only high-speed option on that route).

There are two high speed routes overlapping between Brussels and Cologne. There is both the Paris-Düsseldorf/Dortmund Thalys and the Brussels-Frankfurt ICE.

ICE or Thalys also depends a bit on wether Köln is your final destination or not. If you are travelling Brussel - "somewhere in Germany" I would pick the ICE, as you can then get the whole trip on one ticket.

Other advantage of the ICE is that reservations are not compulsory, so you can use a pass on it, or get a flexible ticket.

I would prefer Thalys as I find the ICEs to be very stuffy until you cross the German border, and I've even felt travel sick a couple of times. I've seen some online suggestions that it's an issue with the air conditioning system not liking the Belgian DC electrical supply.