Watching steins;gate is a must before watching this, and knowing at least some of the plot or premise of chaos;head is also important(visual novel was apparrently good, anime was apparently awful). If you don't intend on watching chaos;head which I don't think you should, and you don't want to play the visual novel, you should read the plot.
They tie in to R;N in the later episodes.
Steins;gate has a very slow start compared to most anime, with the first half of the series being setting, character and minor plot development, with the next half being plot and action.
Robotics;notes goes one step further with like 10 episodes of character and setting development(no joke, like take the first 3 eps of S;G and make it 10 episodes), like 5 episodes of character and plot development, and what will appear to be an action packed final few episodes.
The other problem with the semi;colon animes is that they are VN adaptions. To make the games into a chronological story sacrifices were made, in the case of chaos;head the setting was ruined, in the case of steins;gate everyone bar the main character gets little character development, and in robotics;notes to try and get setting and character development in they throw in several story arcs. But if you can make sense of it all, it truly is amazing.
I flax you not though, Robotics;Notes has the most amazing and *Can'tSayThisOnTV*ed up scene in the history of anime, nay, media, ever. You will know it.