I have been a fan of MST3K for a little under 20 years-I was late to the party, that's for sure. Anyway, in that time this show has become something I truly love and one of the things that actually gives me comfort and makes life worth living. I just recently saw this episode for the first time and while it may not be the absolute best episode, I highly enjoy it.
The most off-putting thing here is that this episode has Erhardt and the original Tom Servo. I am sorry to say it but he has just never felt right to me (which seems to be the case for most fans), so traditionally I have avoided most episodes he is in. That being said, I saw some stills of the bizarre monsters in this movie which look like gorillas wearing space helmets and having no discernible faces (the faces kinda look like "son, you got a panty on your head" but it's tough to tell for sure) and I decided I had to see the episode.
I find this episode to be hilarious. The movie Robot Monster is bizarre and terrible (not to mention the two chapter plays shown before the movie) but still somewhat entertaining and when combined with the riffing the whole thing is more than worthwhile to me, no question. There are plenty of laugh out loud moments as far as I am concerned and the invention exchange between the Mads and Joel and the 'bots is cute. I consider this to be one of the "quieter" episodes, which I think some people my not care for but I love. I have come to appreciate Joel's mellow demeanor more and more over the years. I like some of the older episodes where they did not feel that every single dialogue-less moment of the film had to be filled with jokes (whether funny or not) which, as funny as I find the show all the way to the last episode, really does become a bit of an issue for me towards the end of the series. Sometimes less is more.
Bottom line: I think this episode is a must-see for real fans of the show because it is genuinely funny and the movie itself is so odd. For newbies, this is not where I would recommend starting your MST3K adventures, but once you go down the rabbit hole and decide that the show is awesome and you are into it, be sure to check this one out. :)