Omsk is one of the main stops on the Trans-Siberian Railway route and, along with similar stops such as Krasnoyarsk and Ulan Ude, is tempting for a stop to break up the journey. My suggestion is to stop at only one as it will give the break you’re looking for and its nice to at least get a brief look at what a central Siberian city looks like. But lower your expectations- none are particularly pleasant and should be way down the priority list when it comes for the highlights to be seen on the railway journey. Save your time instead for getting off at Irkutsk to see the gem of the trip, Lake Baikal and, if travelling the Beijing route, the Mongolian Steppes around Ulan Bator
For tips on the overall trans-Siberian Railway experience, including which stops to take, see the travel entry The Trans-Siberian Railway from Omsk to Beijing