The Celestron Omni 6mm is a Plossl design. Traditionally Plossl eyepieces contain 4 or 5 lenses. The Omni 6mm is typical in this regard and contains 4 lenses. The lenses are arranged in a symmetrical fashion, being 2 goups of 2 lenses closely spaced.
The benefits of a plossl include the low amount of glass used in their design. The fewest number of lenses (otherwise known as elements) used in an optical system the clearer and more pristine an image will be. It's a different story when talking about premium eyepieces but as the Omni costs only £30 the rule of less is more holds true.
On the down side as a result of the lenses positioning in a Plossl the field of view isn't enormous. The Omni has a 52 degree field of view which is typical amongst Plossls. As the 6mm provides relatively high magnification (making it best suited for lunar and planetary observation) the field of view may seem frustratingly small especially on an undriven mount. At high magnifications objects such as the moon and planets appear to move rapidly through the telescope, this is caused by the Earth's rotation and can be the source of much frustration in smaller fields of view. A small field means more regular adjustment of the telescope to keep the object centred. This is not only annoying but can actually make it harder to pull out faint details as part of your brain is being held back by having to concentrate on keeping the image centred.
The Omni has 5mm of eye relief which may become uncomfortable, this eyepiece is absolutely not recommended for spectacle wearers. The field stop is very tight and can make it feel like looking through a pin prick, this can cause eye strain and headaches making for an uncomfortable viewing session.
Optically this eyepiece is reasonable for the price, contrast is fair, light scatter is minimal. Practically it's an absolute nightmare to use. Short eye relief and a small field stop conspire together to make for an unpleasant experience. Though the optics are of a quality in keeping with the eyepieces price (or to put another way it's worth £30) the prohibitive physical traits of the eyepiece make it one to avoid.
For a similar price why not check out the Skywatcher range of Plossls, far superior to the Omni and far more comfortable to look through.