Your control with pen (and gel pen, holy cow) is incredible! The concept is very neat and I love your use of color and shape.
The biggest thing that stands out to me is your composition. I feel this piece could be a lot stronger with a more intentional balance (or imbalance) of the positive and negative space. As it is, a vast majority of the color hangs on the right side of the image. This, coupled with the addition of more circles tends to leave the right side of the image far heavier than the left. Even with the inclusion of the pens on the left side, the current imbalance of the image tends to draw the eye out of the right side.
Your specks and swirls add effective visual interest of the piece. Be careful when cropping your image and take into consideration what the negative space (in this case the blue background) looks like. If you think of the colors, specks, and pens as a black block and the background as a white block, it becomes easier to see. See this example. The pens lead the eye in, but due to the weight on the right side, the eye tends to escape when the positive space exits the drawing. Another note- the difference between the negative spaces at the top and bottom of the image doesn't exactly detract from the image, but it doesn't contribute to the image as a whole either.
In future projects (or if you really wanted to go in and modify this one), try considering how both spaces work together to create the image. ^^ This example moves the left side off and the pens further down the page to balance the space, while also balancing the space at the top and bottom. I also added more to the positive space on the left side to give you an idea of what the balance might look like. (The idea is just a suggestion for a potential starting place and I'm sure your idea would look much better.) This balance tends to keep the eye at the center circle, where most of the weight now lies, effectively keeping the viewer engaged in the piece. This other image is another possibility of how it might look if you were to crop the same (modified) image along all sides. It's not necessarily as strong as the previous example, but is another of many possibilities just to give you some ideas.
That being said though, you have very nice, crisp work and you clearly put a lot of time into this. Keep it up! ^^
(From Comment Wars.)