I apologize for the lengthy post in advance, but I thought I would throw this out there as I have heard/read many talk about this subject.
I have about 9 vehicles most of which are maintainers most of the time. I currently own several Battery Tender's, 3 Battery Saver (R2 Battery Saver), 1 Ctek 3300, 3 or 4 Harbor freight trickle chargers, 2+ Schumacher chargers, etc. I also use a lot of Optima batteries (red and yellow top) as well as several lead acid batteries and a Die Hard Platinum.
I went to SEMA last year and ran into the guy from R2 Battery Saver who regaled me with an explanation of how high speed his devices were/are and how they extend the life of a battery (especially a AGM) far longer than anything else out there. I was impressed as it seemed well thought out and executed, therefore I bought 3 of them even though at the time I already had several battery tenders, the Ctek and the HF tenders.
I had a relatively new yellow top go dead in one of the vehicles due to non use and a lack of a maintainer so I thought this would be a good test for the Battery Saver as it supposedly would bring back a dead AGM battery. Well, in that case it did not despite leaving it connected for over a week. I ended up putting a new Schumacher charger with AGM mode on it only to have it get super hot and actually vent off (as in steam coming out it). Needless to say, that battery was toast; I cant blame any device for not helping that one.
I recently (last week) left the key on in my 52 rat rod Dodge PU which drained the 10+ year old red top. I thought this would be a good test for the Battery Saver so I connected it. After 2 days it would say it was 100% but only showed 6 volts. I then removed it and attached the Schumacher AGM charger, it got to 7.8 volts and said it was 100%. So naturally I thought, "this battery is done". I watched a You Tube video from Optima about bringing a dead Optima back in which they used a Ctek charger; since I own one I thought lets give that a try. I connected it and within 3 hours I was at 9.8 volts, 5 hours 11.7 and within about 18 hours 13.4 volts showing a full battery.
The morale of this story (for me) is that Ctek is a great product for AGM batteries and I will be buying more. I cannot bad mouth the battery tenders (brand) as they keep my Harleys topped off as well as my Viper. As for the HF tenders, I think they do work to some degree on a good battery but I do not think they will help a run down battery, for sure not in a timely fashion. As for the R2 Battery Savers, they at are probably ok as maintainers but nothing beyond that. Bear in mind that the Ctek and the Battery saver at both entry level models, so it isn't as if the Ctek was higher level in terms of capability. Yes the Ctek is probably $20-$30 more money (for the base models) but as far as I am concerned far better.
Next one I buy will be a higher end model with desulphation mode (recondition) as I have come to believe that is something I need.
I am sure there are many opinions here about this subject probably beginning with the battery first (many do not like Optima's), however I have been using them for the last 20+ years and find they do well for my applications. Hope this is helpful to someone and since Xmas is almost upon us, a Ctek charger might be a good gift (to you
Just a last note; I don't work for Ctek nor am I in anyway affiliated with them, I just own a pile of cars that need batteries maintained and charged from time to time.