I use Numista and have 1,949 coins cataloged in it right now. There are some areas where it is really useful.
1. Searching, if you are not sure where a coin is from you can enter part of the legend or a date and quickly narrow down the possibilities.
2. Pictures, there are pictures for a lot of coins that are not in the World Coin catalogs. Some of the commemortive stuff takes a while to get into the print catalogs.
3. Upload ability, if you have a coin that isn't in the numista catalog, or doesn't have a picture, you can add the entry yourself and make it easier for those who come after you.
4. The dashboard map. I really love the map and the way it colors the countries based on the number of coins you have. That map is one of the main reasons I started using numista.
5. Mobile access. Their mobile page is not the best but you can use it to see if you have a coin while you are at a coin show or shop.
I really love numista, but there are some problems with it. So keep in mind that I am saying it is really good but there are some really bad problems.
1. The forum. It is no where near as nice as this forum. There are a lot of really annoying people, and a large number of jerks. The moderators of the forum are not doing their job.

2. There are a lot of internal politics on the French side of the forum (it is a French site). People think Americans are jerks (and they are), but French people are a whole different type of jerks.

What I do is use numista for the catalog features and then use this forum. The people here are way more friendly, professional and knowledgeable.

3. Countries.
a. The England, Great Britain, United Kingdom separations are a mess. I have no clue why they do it that way.
b. Some people have vendettas against certain coutries and try to remove them. Or people have their own pet countries and try to add them.
Some days I log in and find I have gained or lost a country. Taiwain is a good example, there is a faction that thinks it should be part of China. Hutt River is another one, it is listed as a full blown country even though it is not a real country. Someone removed it for a while but it came back.
There are also problem with how a country should be named. Some people think they should be named in the original language, others say they should be named as they are known in someones local lanuguage. (e.g. should it show up as São Tomé and PrÃncipe or St. Thomas).
They also have a tencancy to collapse countries in on themselves, (e.g. Indian Princely states, German states, China, etc.) Which is fine but it is inconsistant. As already brought up French Emergency coinage has it's own catagory (French Cities), but German Notgeld goes under Germany.
If it is not a modern country (e.g. last 100 to 150 years) numista is not very good. The Ancient and Medieval sections are a mess.
c. Another problem is the tokens section. There is a large number of people who see no different between a Chuck E. Cheese token and Hard Times tokens, Civil War tokens, Condor tokens, etc. There are other countries that issued 'notgeld' during the war that circulated as legitimate coins. Most all of these items just get tossed into a tokens catagory.
d. Fantasy coins (Coins produced by private companies for countries that do not exist, or for areas that do not have a currency) also have a tendancy of slipping in. The Galapagos coins are a good example. They are part of Ecuador and use Ecuadorian currency, but a private company minted some coins for them in an attemt to fool some unsuspecting collectors. It is sad because I really wish there were a Galapagos coin with a giant tortoise depicted.
Just my thoughts on numista.