AC Conditioners?

Interested in a used AC Conditioner but don't know much about them. Can anyone tell me if they lose functionality as they get older? Do they where out? Do they not filter as well after protecting from some kind of surge? What are the considerations and what questions should you ask? Appreciate any info anyone can provide....
I bought one. A Monster HTS1000. My preamp buzzed when plugged into it. I got my money back and no more Monster for me. I found better results going to the hardware store, getting a six foot 3 wire 16 ga extension cord. Cut the jacket of of it and braid it. You can get new plugs for the end to plug it into the back of your amp. Believe it or not this helped enormously.
If you are looking for used stick with quality. Assuming you do not want to spend big big bucks I can recommend Chang Lightspeed or Vansevers. They are both built well to last. I have a Monster that does a good job with cleaning up my TV picture and video. Colors sharper. Not good enough for good audio.
I think it depends on how you define "functionality". It is my understanding that the filtering qualities of a "line conditioner" will remain indefinitely. I have been told, however, that the ability of the conditioiner to protect from power surges does diminish over time (which is also darn important) A garage manufacturer of such products informed me that the part (I can't remember the part name) that handles surge protection fails with age and that even computer type cheapy power strips utilize the same technology as is in most hifi conditioners. This reminds can get surge protection insurance from you local power company(cheap) in case of disaster (because home insurance would not cover electrical surge damage).
I too have had good results with Chang Lightspeed. They have different models depending upon your system/needs ranging from quite affordable (the 3200) to somewhat more expensive (6400 or 9600 or 9900) but none considered high-priced. Changs are also nice because they don't constrict dynamics. The internal M.O.V. (metal oxide varistor) transient protection devices are good ones that don't degrade. Cheaper (low power rated) MOV's can deteriorate if they're hit enough times with large enough transients; however this is not usually an issue unless you take a direct lightening hit. Mike VansEvers has some nice stuff & it's tunable for your rig too. Have never tried them, but have read a lot of good things & Mike is a good guy to deal with (he worked with me on some other power issues). His recommendations regarding affordable way to fabricate a dedicated power line worked wonders for me! See this thread for details
I've taken flack for my opinion on power conditioners in the past, but what the hell, here I go again. There's no doubt the quality coming out of the wall is garbage, no need for debate there. One of the distinguishing feaures of most high end gear is attention to detail in their power supply design. Massive supplies, with no holds barred parts quality, deal with the shortcomings of your local electric company. Conditioning the AC so these sophisticated supplies essentially sit idle is redundent at best, and certainly a waste of money. I'd invest in a conditioner if I had mid-fi gear, but that really doesn't apply to most of us that frequent this site. Time to put the body armour on and dodge the bullets...
I use a Monster HTS2000 power conditioner with good results. I live in LA and have a very dirty power supply. It may restrict the dynamics somewhat but it is hard to tell because without it the background noise level is much higher, and there is as well a very noticable drop in detail. My system sounds better and more musical with the Monster and it was only $125.00 mail order. I also run my amp through one of the special "amp" outputs (that claim to not restrict power/dynamics) with better results than straight into the wall. I guess that it all boils down to which is the worse of two evils in your particular situation. My gear is not expensive but is also not midfi.
I live in Chicago and have the lesser of the evils syndrome as well. I use a newcastle conditioner and the noise it reduces is nicer to listen to than the garbage on the line. My system is about 25K retail, so I do not think I am at a Mid-FI level. I mainly use the conditioner in case of spikes and browns. In the spring and the fall I do plug directly in the wall between midnight and 4am to realize what my system can do -- and it does some things better on the conditioner and others better on the wall outlet. Trade-offs galore...
There's a Cinepro 20 Line Balancer for sale right now at less than 50% of retail, on Audiogon. I have one, and it's far better than the API Power Wedge I now have in basement storage. If you can afford $850 (a guess from memory), you can't do better. An idiotically lukewarm Stereophile review has kept this conditioner from being as popular as it should be. It splits 120V into 60V+ and 60V-, a brilliantly simple idea, which leads to the lowest noise floor ever.
Post removed 
I have used the Noisetrapper, Power Wedge I, improved power Wedge 1, the Power Wedge 116, and they all hummed. The newer ones aren't supposed to. They decreased the noise in grundge out of the AC, but they increased audible noise from their transformers. The Chang 6400 was next. Better, quieter, but tended to thin the body of the music. It sounded better in my friends system then in mine. I've settled on a Monster 2500(2000 in a metal chassis) and it does a lot to cleaning a midbass hump I was chasing in the system. The highs were also improved with the bass tighter. Dedicated lines are a must. I use two with 12ga romex underground in conduit with 20 amp breakers. Listen before buying.
Re: twisted (or braided) conductors - this relates to something about the magnetic flux fields cancelling each other. You'll find that some upgrade AC cords are made this way too. If you run a dedicated line this way (I did) you must run this twisted wire bundle in a conduit (or in 3/8" Greenfield if code permits - much easier to work with).
Monster is entry level. If you are worried about money then go wiyh Change,which is good,but if you are serious,go with PS Audio Power Plant. I have $30,000 in my system and for $895.00 it was my best move. Good Luck!!!