What about?

SELECT userid,MAX(adate),(array_agg(value ORDER BY adate DESC))[1] as value
FROM YOURTABLE
GROUP BY userid

This is Postgres. I'm not familiar with Oracle. The best I could do was

SELECT 
  userid,
  MAX(adate),
  SUBSTR(
    (LISTAGG(value, ',') WITHIN GROUP (ORDER BY adate DESC)),
    0,
    INSTR((LISTAGG(value, ',') WITHIN GROUP (ORDER BY adate DESC)), ',')-1
  ) as value 
FROM YOURTABLE
GROUP BY userid 

Both queries seem to return the same results as the accepted answer. See SQLFiddles:

1. Accepted answer
2. My solution with Postgres
3. My solution with Oracle

If you're using Postgres, you can use array_agg like

SELECT userid,MAX(adate),(array_agg(value ORDER BY adate DESC))[1] as value
FROM YOURTABLE
GROUP BY userid

I'm not familiar with Oracle. This is what I came up with

SELECT 
  userid,
  MAX(adate),
  SUBSTR(
    (LISTAGG(value, ',') WITHIN GROUP (ORDER BY adate DESC)),
    0,
    INSTR((LISTAGG(value, ',') WITHIN GROUP (ORDER BY adate DESC)), ',')-1
  ) as value 
FROM YOURTABLE
GROUP BY userid 

Both queries return the same results as the accepted answer. See SQLFiddles:

1. Accepted answer
2. My solution with Postgres
3. My solution with Oracle