I went to the top of Jungfrau in January 2007 (almost the same temperature levels as February) and kind of liked the adventure, as it was snowy and stormy. Although most part of Switzerland is mountainous, they have pretty good infrastructure. This applies to Jungfrau too, where they have bore into the mountains and created Cog railway. This is one of the reasons why the trip to Jungfraujoch is expensive.
Switzerland (or any other cold country) is worth to explore in both summer and winter, as the nature and landscape look totally different. Also, the advantage of visiting Switzerland in winter is that it is less touristic/commercial when compared to summer.
If you plan on visiting Jungfraujoch in winter, start as early as possible, as the train service closes at around 4 PM, coz it gets dark after that. Also, check the weather forecast before doing that.
EDIT: As for mountains, they are not closed in winter, but most of the mountain passes (For Ex: Susten Pass, Great St. Bernardino Pass, etc) will be closed in winter.