Multi-effect and single effects pedals all have their place. I tried a ZOOM 505, which was a lot of fun, but tended to be pretty noisy with my Super Reverb. The low cost multi-effects also tend to keep effects in the same position in the signal chain. For example, you might not be able to move the distortion after the chorus. More expensive multieffectors will allow you to rearrange the signal chain. I also found that the Zoom effects tended to be a bit limited when used on their own - each individual effect was not as versatile as single pedals. Multi-effect pedals may also start to drop certain effects that require a lot of storage (reverb and delay) if you use many different effects.
With single effects, your sound has a lot of versatility. You can arrange/rearrange effects and sculpt your tone very precisely with easy-to-use knobs. On the down side, most single effects pedals are not programmable for different degrees of sound. The effect is either on or off. If I need to change sounds between songs,I have to bend over and twist the knobs.
I currrently use an Ibanez TS9 reissue and a Danelectro Chorus and Delay. I am thinking of trying one of the new generation of multi-effect pedals, such as the Korg AX-1000 or Boss GT-3. These units are very versatile and I have been very impressed with the sounds from the GT-3. You must spend some time with any multi-effect unit though - they are considerably more difficult to program even though you can get superb sounds.

[This message has been edited by Jon68 (edited 08-18-2000).]