People with opinions and no proof to back them up bother me. Warmogs is one of the most viable defensive items in the game right now from a purely defesnive stand point against any team with a semblence of balaced damage output. I know this because i have written a program that calculates every major defensive items EHP for every possible in game scenario you can think of.
Based on a baseline scenario of an end game carry with no defensive items...I made a graph of the most common endgame items they might take to help them survive and their net EHP gains from those items. This graph shows the EHP gains for each item when that carry starts with a baseline of 80 AR and 30 MR before purchase. Thier starting HP is listed at the bottom of the graph, and accounts for variation in champions HP totals at level 18, such as Cho'Gath, but most champs will have between 1700-2000 HP at level 18 when excluding other non-defensive HP items like Frozen Mallet.
You can clearly see from the graph..WM is one of the top choices against balanced damage. I listed a fully farmed one as WM*, also in green. The break even point for a fully farmed WM* is well above the normally achievable HP totals for most carries, even with HP items, and even for most champs in the game. The fact that it is now easier than ever to farm it up, due to enemy kills AND assist adding 10% of the bonus each time...means even as a late game purchase it grows quickly. A few team fight victories and you can easliy have 50% of the buff right there. Add in some minion farming and its just too easy to max it out not to consider the fully farmed potential when discussing it as well.
Adding additional AR/MR before calculating the EHP of the item only moves it higher and higher above the other items listed. For instance, if you traditionally take Merc Treads or have scaling MR that would be raise it to 50-80MR instead of the baseline of 30MR i used...WM gets even better than the listed numbers show here. The only thing that lowers its effectiveness as an endgame item is taking other items that already add significant HP, and even then it probably surpasses most other items after it is farmed up.
The 2 major exceptions to WM being the best defensive item in game are when teams are focusing in one damage type or the other on you. In that case RO(physical) and BV(magical) both outshine a fully farmed WM at about 2,500 HP before purchase, with significant gains over WM the higher your HP total is above the 2.5k mark.
The other thing to consider is non-defensive attributes that items often provide. WM only adds Hp5 in additon to the massive number of HP you get. Some champions benfit more from other stats, and thus find the added EHP of Warmogs less effective than slightly less EHP when paired with other beneficial stats from a different item.
Finally, another item worth mentioning is AoL...which for its cost, offers incredible EHP value and team support to boot. It kind of gets lost in the graph, but its EHP/gold are second to none. As a purely defensive item however, when considering flat EHP totals...a WM is a very solid choice...and the fully farmed WM* truely has no equal.