Most disappointing purchase was a pair of Spendor S3/5 speakers. Technically they just made sense to me before I heard them; I was sure they would be the thing to keep me off the speaker-buying treadmill. They were OK, just nothing exciting. They moved on pretty quickly.
Bigger disappointment was hearing B&W 602 S3s. I was new to all this, and somehow got stuck in the B&W threads on AVS. I found a semi-local dealer, took off work, and was ready to go buy some amazing speakers. They were in a well-treated room, hooked to McIntosh electronics, and just sounded awful to my ears. I'm sure I was expecting too much. I've always wanted to sample the higher end of the B&W line, to try and get that sound out of my ears, but I have not ventured back.