I went from Slicehost to Linode a few months ago. Linode has faster boxes. I did a one week performance comparison between Slicehost, Linode, and Prgmr (which I should publish...). Linode won by a large margin.
+1 linode. Been using them for about 4.5 years. Solid service, knowledgeable staff, consistent improvements. "Just works(tm)".
Very positive experience here. Also, they offer 32bits slices, while slicehost only offer 64bits.
I've had a few VPS off them for small projects over the last year and it's all worked great.
Oh, in that time, they've upped my ram and my storage space multiple times, FWIW.