I've been to Sde Boker twice. The first time was to go and see the famous tomb of Ben Gurion. The tomb may not be important to or have a particular significance for all travellers, but even if it's not important to you, the site is worth a visit because of the magnificent view which you get from the memorial. In fact, the tomb faces the desert which the country's founder loved so much.
The second time I went to Sde Boker it was completely by chance. Night was falling and I was looking for somewhere to sleep and I went there because it was the only place for miles around that had rooms free. Once I got there I realised that Sde Boker was quite a dynamic little kibbutz: there is a Bedouin market where everybody goes shopping, a little bar with a terrace where all the young people meet up, one or two craftsmen, etc. In brief, the unexpected visit to Sde Boker was definitely the best. If you get the chance, I recommend you spend a night there during your trip to Israel.