Do you still use MyISAM or prefer Aria storage engine?
It is very interesting this question would come up because a similar question was asked back in January 2011 ( When is the right time to use MariaDB instead of MySQL, and Why? ). Giving thought to that 16-month-old question and how I answered it in April 2011, here are my answers to your immediate questions:


  Why still using MyISAM ?


MyISAM still has features that are unique and useful for small datasets and read slaves in HA setups


Deadlocking is impossible in low-write, heavy-read environments


MySQL MyISAM Locking question
Benefits of table level locking

Converting to Fixed Row Format creates bigger tables but increases query performance in terms of 


character string comparison/compression : What is the performance impact of using CHAR vs VARCHAR on a fixed-size field?
read/write speed : Which is faster, InnoDB or MyISAM?

Full Faith and Confidence


InnoDB has become the de facto standard of Transactional Storage Engines for MySQL. XtraDB is being accepted and used as well among the Percona Server community. Full faith and confidence seems to rest with InnoDB over XtraDB as present. This could easily change over time as more recognize XtraDB's superior features.
The same equally applies to MyISAM vs Aria. Full faith and confidence currently rests with MyISAM. An example of those who do not have Full faith and confidence are Drizzle users (Drizzle have abandoned MyISAM as an Optional Storage Engine and relegated it to temp table use only)




  To changes storage engine from MyISAM to Maria should it be a problem? (lose index or something)


If you mysqldump the data and the schema into two separate files, you can always import data into table with either the MyISAM or Aria storage engines. Just keep around the data only mysqldump to be sure.